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1966 Owners Manual

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         The VW Carburettor

By Rob Boardman

 

~~~Following are links to various topics discussed under the

general heading of "carburettor."

The Carburettor - General Description and Discussion

--

Single-Port and Dual-Port Cylinder Heads

More Technical Detail 28 Series

Carburettors 30 Series

Carburettors Brosol H30/31

Carburettors 34 Series

Carburettors The Pierburg

34PICT/3 Carburettor

Carburettor Adjustment - Adjustment of "Single-

Screw" Carburettors Adjustment of "Double-

Screw" Carburettors Miscellaneous Topics

Related to Carburettor Tuning

Carburetor Overhaul

(Disassembly and Cleaning)

28 PICT-1 & -2; 30 PICT/1 Carburettor Overhaul

34 PICT/3 Carburettor Overhaul

Related Topics         (In alphabetical order)

Accelerator Accelerator Cable Accelerator Pump Alternator/Accelerator Pump

Linkage Interference Air Cleaner Air Inleakage Automatic Choke Carburettor/Distributor

Matchup Dual Carburettors Float Needle Valve Fuel Line Input Problems Flooding Intake Manifold Jets Throttle Positioner Throttle Shaft Bushings Vapor Lock Welch Plugs

 

 

Carburettor Interchangeability Miscellaneous Notes/Questions and Answers

~~~

The Carburettor

 

The VW Carburettor

(In this case, a 34PICT/3 model)~~~

Brosol 30/31 Carburettor

(Fairly typical of VW carburettors)~~~

Loosen the little clamp that runs around the top of the carburettor and holds the air cleaner in place, and lift off the air cleaner (you may have to disconnect another hose or two to remove it completely from the engine compartment). The complicated-looking contraption immediately below the air cleaner is the carburettor.

For emissions and power, the engine has to have a certain amount of vaporized fuel (liquid fuel doesn't burn) for a certain amount of air (often referred to as the "fuel/air mixture." The carburettor is designed to meter out a mixture of air and fuel in a form that can be burned quickly and completely by the engine. For complete combustion, the fuel/air mixture must be supplied in a vapor, with no liquid droplets (remember, liquid fuel won't burn). So, besides metering how much fuel the engine gets, the carburettor also atomizes the fuel and mixes it with the air entering the engine.

The role of the carburettor is complicated by the fact that air and fuel

have different viscosities. Since air and fuel do not flow the same, the metering of fuel is NOT LINEAR. What this means is that you can have correct metering at some engine speeds but not at others. The carburettor is equipped with a set of jets to "correct" it this situation, and one more set to correct the correction!

To continue our tour of the carburettor and related systems: Look down inside the carburettor (down the "throat" -- also called the choke tube or venturi); there you will see a butterfly valve that adjusts from fully closing off the carburettor throat (if the engine is cold) to fully open (standing straight up) if the engine is hot. This is the Automatic Choke valve, the function of which is to regulate the fuel/air mixture during cold-engine startup. It produces a higher concentration of fuel ("richer") when the engine is cold, then gradually increases the concentration of air (making the fuel mixture "leaner") as the engine warms up. Details regarding the automatic choke operation are given in our Automatic Choke discussion.

At the bottom of the carburettor is a second butterfly valve, the Throttle Valve (you can't see it, but trust me) which opens to supply air and fuel to the intake manifold and on to the engine. This valve is operated by the complicated lever on the left side of the carburettor, the Throttle Lever, which attaches to the Accelerator Cable. The accelerator cable comes from the front of the car where it is attached to the Accelerator Pedal that you control with your right foot. On the throttle lever (back at the carburettor) there is a return spring that returns the Throttle Lever Screw to rest on the Stepped Cam on the left side of the carburettor. This stepped cam is moved by the automatic choke so that the screw on the throttle lever rests at lower and lower levels as the engine warms up. This assures that the engine has a high idle speed when cold (or it would stall), but the idle speed reduces to normal as the engine warm up. The engine running without your foot on the accelerator pedal is called .

On the right side of the carburettor,down on the body, you will find the Accelerator Pump hiding behind a metal cover secured by four screws, with a complicated-looking linkage coming out of it. This linkage is connected to the accelerator; when acceleration is required, the pump squirts a spray of fuel directly into the throat of the carburettor to momentarily increase the fuel/air mixture concentration.

Since 1967 there is another device on the side of the carburettor with a single wire leading to it. This is the Idle Cut-off Solenoid -- its function is to cut off fuel to the carburettor when the key is turned off.

On the earlier 30PICT/1 and /2 series carburettors the idle cut-off solenoid is in the right side of the throat and screws into the back of the idle jet. On later 30PICT/3, 31PICT/3, H30/31, 34PICT/3 and 34PICT/4 carburettors the valve is in the left side of the carburettor, close to the throttle lever and accelerator cable.

Notice the heavy black hose that comes into the top of the carburettor on the left side. This is the fuel supply line. It directs fuel into the bowl of the carburettor through a Float Needle Valve (inside the carburettor bowl -- you can't see it without taking the top of the carburettor off). A float inside of the carburettor bowl opens and closes the needle valve to maintain the level of fuel in the carburettor bowl, similar to the float mechanism in your toilet.

Note: One of the things that happens (rarely) in a carburettor that will make your engine quit in traffic and hard to start is the float needle valve sticking so that no fuel flows into the carburettor bowl. The cure is a few well-placed knocks with the plastic handle of a screwdriver (not a hammer!) on the top of the carburettor bowl. If the car makes a habit of this, its time to rebuild the carburettor and replace the float needle valve, which is included in most carburettor overhaul kits.

On the bottom of the carburettor bowl on the left side is a large nut. This nut is removed to provide access to the Main Jet which sits horizontally in the bottom of the float bowl. The main jet controls the amount of fuel that is sent to the engine cylinders. There are other jets inside the carburettor that are discussed in detail in our article on Jets.

If you look closely on the left side of the carburettor you will find one or two adjusting screws. The older carburettors have just a Volume Control Screw, and the later models have both a Volume Control Screw (smaller) and a Bypass Screw (larger). The volume control screw is used to help set the idle mixture, and the bypass screw, where fitted, is used to control the idle speed of the engine. We use these screws to "tune" the carburettor. Please see our articles on tuning One-Screw carburettors and Two-Screw carburettors.

Follow the fuel supply line down from the top of the carburettor to just below the Distributor (the round orange or black thing with five heavy wires protruding from the top of it). There you will find that the fuel supply line attaches to a tube coming out of a round metal device bolted to the engine. This is the Fuel Pump. There is another connection point on the fuel pump with a metal or rubber line

attached to it. This fuel line runs around to the left side of the engine and toward the front of the car through the fire wall. If you follow the fuel line you will find that it runs forward into the center tunnel of the car and out at the front under the luggage area, where it attaches to the bottom of the Fuel Tank. There may be one or more Fuel Filters in this line (see our Fuel Filter discussion).

Now return to the engine compartment. The bottom of the carburettor is bolted to the top of the Intake Manifold, a metal tube which splits right and left and carries the fuel/air mixture from the carburettor down to the sides of the engine, where it attaches to the Cylinder Heads on either side.

If the intake manifold splits in two on either side, you have what is called a Dual-Port Engine; if it continues as a single pipe from head to head, your engine is Single-Port.

 

Single-Port Dual-Port(To clarify: In the single-port design the port splits INSIDE the head to provide the fuel/air mix to each intake valve. The twin-port (dual-port) heads have two holes in the cylinder head, one for each intake valve, since the split occurs in the intake manifold itself before it attaches to the cylinder head.)

~~~

Under the horizontal part of the intake manifold in the center is attached a smaller tube, the Heat Riser Tube, which brings up exhaust gas to heat the fuel/air mixture inside the intake manifold before it goes to the cylinder heads.

Inside the heads are the Intake and Exhaust Valves. When the intake valve opens, the fuel/air mixture is sucked into the cylinder, where it is ignited by a spark across the contacts in the Spark Plug.

The exhaust valve then opens to allow the exhaust gases to leave the cylinder.

~~~

More Technical Detail AboutVolkswagen Carburettors

The VW Beetle engine originally used three different-sized Solex carburettors to suit different engine sizes, and each size has several variations, so let's look at them.

Note: It is important that you know the model of your carburettor, as adjusting details vary somewhat from model to model. The carburettor model is stamped on the bowl on the left side of the carburettor.

Until 1972 the Type 2 (Bus/Kombi) also used the engines noted below, and in fact they got the larger size engines before the Beetle did. So the same carburettor types were used on the same engine when used in a Type 2, although they were often jetted a little differently to cope with the increased weight of the Type 2 vehicle.

1. The 28PCI, 28PICT and 28PICT/1 carburettors were used on the 36 and 40hp 1200cc engines from 1951 to 1965. These have a 22.5mm venturi (throat), which provides airflow characteristics which suit the 1200cc engine size. These carburettors are no longer made, but can be replaced by the slightly larger modern replacement Brosol H30/31 (see below), which will provide a small increase in hp for the 1200cc engine.

Do NOT use a 28 series carburettor on the 1600 engine - it's way too small and although it will run, the car will lack power. And since these smaller carburettors don't have a power jet they run the engine lean at high airspeeds through the carburettor. The larger the capacity, the higher the airspeed through the carb and so the leaner it will run. These carburettors have a 22.5mm throat.

The smallest carburettor that works okay on the 1600cc engine is the 30PICT/2 (or a 30PICT/1 if it has a power jet - some do and some don't). These have a 24mm throat. The modern H30/31 works well too - it has a 25.5mm throat. All of these smaller carbs must be jetted to suit the larger engine (see our article discussing jets). They sit on the twin port manifold with a

30/34 adapter plate. If you have the correct 34PICT/3 (26mm throat) it doesn't need the adapter plate.

2. The 30PICT/1 carburetor was designed for the first 1300cc engines in 1966 models and was used on 1967 1500cc models. Then came the 30PICT/2 in 1968/1969 for the 1300/1500cc engines and the 30PICT/3 for the US-only 1970 1600cc single-port engine. These all have the same sized venturi (24mm), which allows a larger airflow than the earlier 28 series - which suits the larger capacities better. The /1, /2 and /3 30PICTs have increasingly complicated fuel circuits for better mixture control. Any of the 30 series (30PICT/1, 30PICT/2, 30PICT/3 and modern replacement H30/31) will work fine with the 1500cc and 1600cc engines.

Someone wrote to ask -

Can I use my 30PICT/1 carburettor on my 1600cc dual-port engine?

Rob responded -

You can use SOME 30PICT/1 carburettors on a 1600cc engine. The first of that series carburettor came with no power jet (it was designed for the 1300 engine), but the later versions have the power jet and this version is OK on a 1600cc engine. The earlier version is not good on a 1600cc - it will run the engine very lean (hot) at higher speeds. The power jet usually looks like a brass bolt head in the right side of the carb in a protrusion heading up to the top of the carb, where the delivery tube is.

The 1970 1600cc has a B in front of the engine number, and was a single port engine. The first dual port 1600s came in 1971, and have a double letter in front of the engine number, AF or AD for that year.

The H30/31 Brosol carburettor can run the 1600cc engine OK, but it MUST be jetted correctly. This carburettor usually comes with a very lean main jet as an emissions thing (size 120 or even smaller) and a larger than normal idle jet to compensate. On a 1600cc the main jet should be a size 127.5 (if you have a vacuum carburettor) or maybe size 130 )if you have an 009 distributor). The idle jet should be a size 55, and power jet size

65.

The air correction jet size is very important when using a smaller carburetor on a larger engine. This jet stops the carburetor going over-rich at medium-high throttle. The 1600cc DP can flow more air than the 1600cc SP and smaller engines. And since higher airfow means a higher vacuum in the throat, this sucks in more fuel for a richer mixture, so you then have to enlarge the air correction jet to match the higher airflow. If not, the engine will run rich at medium-high throttle. The air correction jet should be about size 125.

Later twin-port 1300s (after 1971) have a 31PICT/3 carburettor, which has a 25.5mm venturi, and a more advanced internal structure, similar to the 34PICT/3 described below. All these 30 and 31 series carburettors ( 1200cc, 1300cc, 1500cc, and 1600cc engines) can be replaced by the modern equivalent Brosol H30/31, so long as it is jetted to suit the engine size (airflow). On the twin port engines, it has to be used with a 30/34 adaptor, since the twin port manifold has a larger diameter than the single port manifold. (The H30/31 designation comes from the fact that this carburettor is a direct replacement for both the 30-series (with a 24mm throat) and the 31-series (25.5mm throat) used on the post-1971 1300s in Europe etc.

Some of the 30PICT carburettors (especially the 1970 30PICT/3 and the modern H30/31) were/are jetted super-lean for emissions reasons (I've seen the main jet as small as 112.5). Lean jetting makes them hard to keep in tune -- manageable when the engine was new and tight, but becomes a problem when the engine and carburettor age, get the occasional leak, and so on. When using the H30/31 on a 1600cc single-port engine with a vacuum distributor, try a 55 idle jet, 125 main jet and 125 or 130 air correction jet. Leave the 65 power jet (right side) as-is - that's about the right size.

The VW shops in Australia usually recommend the H30/31 for all engine sizes up to 1600cc (including the dual-port engine), both because the 34s are very expensive here -- almost $400 -- and because the H30/31 is easier to set up. The H30/31 also tolerates the 009 distributor better than the 34 sized carburettor -- less likelihood of flat spots. The H30/31 carburettor has a throat only a mm or so smaller than the 34 anyway, so you should not see any significant drop in power

around town (compared to the larger 34) -- it would probably run out of breath a little sooner at high speed and that's about it.

The development of the 30/31 carburettors is interesting... the 30PICT series has a 24mm throat, and it's final development was the 30PICT/3 used in the 1970 1600 single-port engine (in the USA). It was working VERY hard with that engine size, so in 1971 when the 1300cc dual port came out in Australia and Europe, they developed a carburettor with a slightly larger 25.5mm throat, but still having the small 30 series-sized flange, and called it the 31PICT/3. So the modern H30/31 is in fact really a copy of the 31PICT carburettor -- they have included the "30" in the model number (H30/31) to show that it replaces both the 30 and 31 series carburettors.

You can't buy a brand new 30PICT/2 carburettor, but you can buy a brand new H30/31 model, which is the replacement carburettor for all the 28 and 30 series. It has a fractionally larger throat but he same inlet manifold fitting, so you'd get a tiny increase in hp and would probably get a corresponding tiny reduction in fuel economy. The 30PICT/2 is one of the most reliable carbs Solex ever made, and it matches the 1500/1600cc single-port engines very well.

3. The 34PICT/3 and the California-only 34PICT/4 carburettors have a larger 26mm venturi, which allows the 1600cc dual-port engine to breath better for increased horsepower. These carburettors have more complex fuel metering which allows the carburettor to run a little leaner for emissions reasons. These leaner settings also make the engine harder to tune as it ages (the VW engine really likes to run a little rich), and so sometimes changes to the fuel jets are needed. Two replacement 34PICT/3 carburettors that are available the Bocar and the Pierburg. The new Bocar carburettors almost always have very lean jetting which may need alteration. The Pierburg carburettors come with an X130 main jet and a 50 idle jet.

Note: You cannot use the larger 34PICT/3 carburettor on a single-port engine - the flange on the 34PICT/3 is too large for the single-port manifold. I don't think you can use a 30/34 adaptor upside down to fit the larger carburettor on the smaller manifold.

In summary, there are three replacement carburettors available today -- the Brosol H30/31, the Bocar 34PICT/3, and the Pierburg 34 PICT/3:

The Brosol H30/31 replaces the 28 series, the 30 series and the 31 series, and can be used (with different jetting) on 1200, 1300, 1500 and 1600 engines (either single port or dual port). It's just about at it's limits with a 1600, and so it loses a few hp compared to the larger 34PICT/3 carburettor, but still works well with this engine size. The H30/31 flange fits the single port inlet manifold, and the 1300 twin port manifold ('71+ 1300s). It can also be used on the larger 1600 twin-port inlet manifold with a 30/34 adaptor, so it's quite a versatile carburettor.

The larger 34PICT/3 carburettor is used on 1600 twin-port engines and provides slightly better horsepower than the same engine with a smaller H30/31. It can also be used on slightly larger capacities like the 1641 (87mm cylinders) and 1776 (90.5mm cylinders). Bigger than this and the engine will be under-carbed, limiting any gain from the larger capacity.

Note: In Australia the preferred replacement seems to be the Brosol H30/31, rather than the 34PICT/3. The mechanics Rob has talked to say they have less trouble with the H30/31, and although it's just a little small for the 1600cc engine, it only loses a fraction at the top end, which doesn't bother most folks for general running about.

The Pierburg 34PICT/3 carburettor is designed for 1971 dp and newer upright engines. These have the accelerator pump linkage that clears the alternator, unlike the Brazilian carb that requires alternator grinding. The Pierburg models are supposed to have much better casting quality, machining, and almost zero defect compared to other units. They also have the proper port for SVDA or other vacuum advance distributors. The Pierburg carburettor comes with an X130 main jet and a 50 idle jet, as well as a new choke and idle shut off valve. (Dave replaced the size 50 idle jet with a size 55.) The choke and mixture need to be adjusted after installation, of course.

Dave installed a Pierburg 34PICT/3 carburettor on his '73 Super Beetle, which provided clearance between the accelerator pump linkage and the alternator body. However, Dave's euphoria with the Pierburg carburetor was short-lived. After just under three years of service, the throttle shaft bushings became worn to the point that air was leaking into the intake

manifold.

Throttle Shaft Bushings

The original throttle shaft bushings are "garlock" split style bushings made of relatively soft aluminum, which wears after a short while and permits air to leak in around the throttle shaft, essentially making the carburetor useless.

The following note is from Keith Doncaster (keifernet@hotmail.com), who overhauls carburetors and replaces the throttle shaft bushings for a very reasonable price -

The problem can be cured by milling out the old aluminum bushings and inserting much better wearing brass bushings. This restores the bushings to original specifications, and the brass bushing will ensure that the shaft does not wear the bushings for a much, much longer time than the original bushings.

Here's a picture of Dave's worn out bushings -

 

Worn Out Throttle Shaft Bushings

Later Dave replaced the Pierburg carburetor with a rebuilt German 427-1 model from Keifernet, in which Keith had installed brass throttle shaft bushings. Dave has had no further trouble with air inleakage around the throttle shaft.

~~~

Carburettor Adjustment -- General Notes

Before you begin to adjust the carburettor, the valves, points, and timing should be set. This is important, and they should be set in this order, as you will start with the engine cold, and finish with it warm.

It's important to set the valves, points, timing and to check the choke before setting the carburettor, as they all work together for a smooth running engine. Details regarding these settings are given in the links below.

The following descriptions apply to all types of carburettors -

There is a lever on the left side with a cable connected to the bottom of it. This is the throttle lever; the cable is the accelerator cable.

Note: Before adjusting the carburettor, it is essential that the accelerator cable be properly adjusted. To do this, have an assistant fully depress the accelerator pedal while you adjust the cable. Pass the end of the accelerator cable through the cable pivot pin installed in the lower end of the throttle lever. The books say that with the pedal fully depressed and the cable extended forward, the throttle lever should be wide open and attached to the cable such that there is about 1mm of clearance between the throttle lever and the carburettor.

You may find it easiest to simply note where the clamp goes on the cable end in this position (wide open), then let up on the pedal and make the connection with the system relaxed. Or, if working by myself, I find that I can come very close by adjusting the cable as follows: With the idle screw against the very bottom of the stepped cam, pull the cable back finger-tight and snug down the screw to secure the cable. It takes three hands -- I use my channel lock pliers and hold the end of the cable to the throttle lever while I tighten the screw with the other hand.

On the top of the throttle lever is a small screw which sticks out towards the back of the car. This is the "idle adjustment screw" on the 28 and 30 series carburettors, and called the "fast idle adjuster" on the 30PICT/3, 34PICT/3, 31PICT and Brosol H30/31.

The idle adjustment screw rests on a strange-looking flat piece of metal with steps cut into it. This is the fast idle cam, and works with the choke to give a reliable idle on a cold engine.

The engine must be warm to set the carburettor so that the choke is off (i.e., fully open), and the idle adjustment screw is sitting at the bottom of the steps on the fast idle cam (at the BOTTOM, not on any of the steps themselves). Directly beneath the fast idle cam on the left side of the carburettor you will see (on the older models) a single screw with a spring wrapped

around it. This is the volume control screw, and this type of carburettor can be called a "one adjusting screw" type of carburettor. The later model carburettors also have a larger bypass screw that is used for setting the idle speed - the fast idle adjustment screw and stepped cam are NOT used to adjust the idle speed on these "two adjusting screw" carburettors.

On the side of the carburettor body is a barrel-shaped object, a little larger a pen-light battery, with a black wire connected to the outer end. This is the idle fuel cutoff valve (solenoid). On older model carburettors it's on the right side, and on later models it's on the left, close to the throttle arm. This valve shuts off the flow of fuel when you turn off the engine, to prevent "running on." Be sure the wire is connected and that it runs to the (+) terminal on the coil. (Also attached to this terminal is the black wire that provides power to the automatic choke.) Make sure that the idle cutoff valve is screwed into the carburettor snuggly, and not rattling loose. Don't overtighten it though, it's got a fine brass thread and screws into aluminum -- both relatively soft metals.

Note: You can test the operation of the idle cutoff valve solenoid very easily. Turn on the ignition (don't start the car), and pull off the wire on the solenoid. Touch the wire back onto the connector, and you should hear a clicking sound as the valve inside moves. If you do not hear a clicking sound, check to make sure there is power (12 volts) to the wire (small trouble light, voltmeter, etc.). Replace the solenoid if necessary. If it's not working, you won't get a proper idle, and you'll get rough running at traffic speeds, too.

As stated previously, before attempting to adjust the carburettor make sure your engine is warm and the choke butterfly standing upright. Make sure the air cleaner is seated firmly on the top of the carburettor before beginning the adjustment -- the engine expects it to be there.

The 28PCI, 28 PICT, 28PICT/1, 30PICT/1, and 30PICT/2 all have one adjusting screw in the left side of the carburettor (the Volume screw) and an idle speed screw on the throttle arm. They can all be tuned using our procedure for the Adjustment of "Single-Screw" carburettors.

The 30PICT/3, 31PICT, H30/31 and 34PICT/3 all have two adjusting screws in the left side (the smaller Volume and larger Bypass screws) and a fast-idle screw on the throttle arm. They can be set using the

our procedure for the Adjustment of "Two-Screw" carburettors.

~~~

Carburettor Interchangeability

Someone wrote – In my car (1969 Beetle) I have a 30PICT/2 carburettor. I also have an extra 34PICT/3 carburettor from an old '74 that I parted out. Can I use the 34PICT/3 carburettor on my '69 Beetle?

From elsewhere on our Web page - You cannot use the larger 34PICT/3 carburettor on a single-port engine - the flange on the 34PICT/3 is too large for the single-port manifold. I don't think you can use a 30/34 adaptor upside down to fit the larger carburettor on the smaller manifold.

Rob wrote – If you really do need to get a new carburettor, the readily-available replacement H30/31 carburettor will fit straight on your manifold.

This carburettor has a slightly larger throat than the 30PICT you have at the moment. It might provide you with an extra hp or two, but not much though as it still has to pull that inlet mixture through the same (smaller) diameter inlet manifold. So the inlet manifold itself becomes a limiting factor. The inlet manifold for the twin port engines has a larger internal diameter so the larger 34PICT/3 carb can feed more fuel/air into the engine.

I don't quite understand what you are trying to achieve though – you say the car is running well (except for maybe a slight choke adjustment), and your fuel economy is about right for that engine/carburettor combination In US gallons, 25-28 mpg for the single-port 1500/1600cc engine with 30PICT/2 carburettor. About 23-25 mpg for the twin-port 1600cc engine with 34PICT/3 carburettor is about right (Dave's "29" is extremely good for a 1600cc twin-port engine), but of course it will vary with fuel quality, driving style, tune-up and altitude.

The jets in all Solex carburettors are interchangable - they all have the same thread etc., just the hole size is different. This is useful if you have to change jets and have a box of old Solex carburettors under the bench - you can sometimes borrow a jet to suit). The rest of the carburettor components are different, except for the floats, which are interchangable between the 30, H30/31 and the 34 series

carburettors.

~~~

Another person wrote wondering whether a smaller (i.e., 28PICT) carburetor would help to resolve his dieing at idle problem. Rob responded -

Do NOT use a 28 series carburettor on the 1600cc engine - it's way too small. Though the engine will run, the car will lack power. And since these smaller carburetorss don't have a power jet, they run the engine lean at high airspeeds through the carburetor. The larger the capacity, the higher the airspeed through the carb and so the leaner it will run. These carbs have a 22.5mm throat.

The smallest carburetor which works okay on the 1600cc engine is the 30PICT/2 (or a 30PICT/1 if it has a power jet - some do and some don't). These have a 24mm throat. The modern H30/31 works well too - it has a 25.5mm throat. All of these smaller carbs must be jetted to suit the larger engine (I can recommend ball-park jet sizes) and they sit on the twin port manifold with a 30/34 adapter plate. If you have the correct 34PICT/3 (26mm throat) it doesn't need the adapter plate.

~~~

Miscellaneous Notes/Questions and Answers

Note Regarding Carburettor Designations -

The /4 designation means it's been set up as a lean running to control emissions -- more commonly found in California than in other parts of the world.

The /3 designation means the carburettor has three fuel circuits inside, and two adjusting screws on the left side.

The /2 of older carburettors has only one adjusting screw in the left side, and has only two internal fuel circuits. For example, the 30PICT/2 (1968-69) has only one adjusting screw, where the 30PICT/3 is the same size but has two adjusting screws (used in the US only - in 1970).

Questions Regarding a Sticking Throttle Lever (see

Dave's bottom line below) -

Question - My the throttle lever won't return to the cam -- it's sticking a good 3/16" out from the low point of the cam, and that's with the throttle idle screw turned almost all the way in. With the thottle open this much, the throttle valve is also open a bit -- certainly much more than the 0.004 inch it's supposed to be. Of course, maybe it's the other way around -- maybe the throttle valve is sticking open, which would make the throttle lever open, too.

As you said in one of your notes, if the throttle valve is not seating properly (i.e., 0.004 inch open) it will interfere greatly with the normal IDLE flow -- i.e., fuel will be flowing through the transition ports higher up -- which I assume would make the engine run faster.

Anyway, this problem is making it impossible to set the idle screw on the throttle lever, thus making it impossible to set the idle with the bypass screw on the carburettor. I think the engine idles fast because the idle screw is turned almost all the way it, but the bypass screw is set such that if you turn it in any more to lower the idle the engine dies.

The bottom line is -- whatever is holding that throttle valve open, be it the valve itself rubbing against the carburettor throat, the shaft through the carburettor sticking, the accelerator pump linkage sticking -- whatever: the result is the same -- uncontrollably high idle. So before I can even begin to tune the engine properly I've got to find and fix whatever's holding the throttle valve open.

Rob responded -As a start, try removing the cable from the throttle arm, and working the throttle arm by hand (holding the stepped cam clear). You might just feel something if you are lucky. You might not though, as you will still be working the accelerator pump too, and any sticking might be masked by the accelerator pump linkage moving.

Have a look at Bentley "Fuel System" Figure 5-1 (Section 2 of the book). It shows a cutaway of the carburettor in good detail. The right side of the butterfly sits against the idle ports (one right opposite the edge of the butterfly, and three narrow transition ports higher up). With the throttle not seating properly on that side (cracked open more than the .004 it

should be), it would interfere greatly with the normal idle flow in the drillings around the volume screw and idle cut off.

Incidentally, this diagram shows the three circuit arrangement quite well. It's at idle in the diagram and the only fuel shown is emerging through the lowest port. Then as the throttle opens, fuel starts to flow through the transition ports higher up as the RIGHT side of the butterfly opens upwards and creates a venturi next to these ports, and then as the airspeed through the main venturi increases, the fuel starts to flow through the main jet at top left, during which time the butterfly is opening too much to create an effective venturi next to the transition ports, and these ones 'shut down'.

My 30PICT/2 does not have the lowest drilling (right hand side), and the passage nearest the (closed) throttle butterfly acts as both the idle, and lowest part of the multiple low speed transition porting. There is no bypass screw on mine.

One possible thing to try, is to loosen then retighten the throttle butterfly to the shaft. Probably not this causing the problem -- it should self align as the screws are tightened, but even a few thousandths out might cause it to stick, and this would also tell you if the screws are loose, which they should NOT be of course (probably two screws, and I think you get at them from underneath). Also look and the sides of the throat where the butterfly moves. You might be able to see bright 'rub' marks if the butterfly itself is binding on the sides, or on the outer edges (near where it closes).

Dave's Bottom Line Regarding His Sticking Throttle Lever Problem -

Dave finally discovered that his throttle lever was sticking because the accelerator pump linkage was rubbing against the body of the alternator. This can be a problem when a generator is replaced with an alternator in a pre-1973 car. Dave's car is a 1973 model, but the engine is 1971. The PO who replaced the original '73 engine with a '71 simply put the original ancillary equipment back onto it, not realizing (and apparently never discovering) that the larger-circumference alternator was going to interfere with the operation of the accelerator pump. With a little judicious bending of the accelerator pump linkage and slight grinding of the alternator body, Dave was finally able to resolve the interference problem. The rebuilt German

carburetor from Keith Doncaster does not have this interference problem. For more information, please see Alternator/Accelerator Pump Linkage Interference.

Question Regarding the Throat (Venturi) Size -

Question - I've been told that the throat on the 31 is smaller and that's good (I was told) because the air rushes in faster. Any comment?

Rob responded - Well, yes, that's sort of right. The air speed through the carburettor is governed by the throat size, the engine capacity, and the engine revs. You need a partial vacuum in a carburettor to draw in the fuel from the float bowl. Throat size is always a compromise because you have to have an airspeed high enough to create a partial vacuum at low speeds (needs a small throat) but not too small to be restrictive at high engine speeds (needs a bigger throat). The engine would run on a carburettor designed to run a chain saw, but only at idle revs! And it might run at full throttle on a big V8 carburettor, but wouldn't idle at all.

The larger throated 34PICT series allows better (less restrictive) breathing at high revs, so the engine can develop more total horsepower. But it also has to operate at idle and cruising speeds. So the 34 copes with this by using 3 fuel circuits - idle, low speed and main. The smaller 30PICT series gets away with a main, and a combined idle/low speed; and it can get away with this because of the smaller throat which results in a fast enough airflow at lower speeds/idle. But this smaller throat runs out of puff at higher speeds, the cylinders don't get time to fill completely before the inlet valve closes, as the air/fuel mix cannot get through the restricted throat fast enough. So my 1600 with smaller 30PICT/2 carburettor makes about 55-56hp, where your 1600 with larger 34PICT/3 should make about 60hp (I think it was 60 for the '71 engine).

That's why (those who race their VWs) are always talking about dual 40mm Kadrons, progressive Webers etc. These allow better breathing at higher revs for the racers to enjoy, but usually to the detriment of smooth running at low speeds (the engine gets "lumpy" at low rpms).

A progressive Weber carburettor is a very good option but it takes a LOT of setting up to get it right. These have two throats,

a smaller one for idling and gentle cruising about town, and a larger throat which only opens when you step on the gas. The Japanese cars used to use them a lot. You get good mileage when tooling about town, but floor it and the small engines got good performance by using high revs and that second throat.

The 31PICT will certainly run okay on (the 1600 dp) engine -- the first 1600 VW was the US 1970 model, which is virtually identical to my altered 1500. It had 1600 cylinders, with single port head and 30PICT/3 carburettor. Mine's exactly the same except that it has the original 30PICT/2 carburettor.

The 31PICT is a direct replacement for the 30 series, with (I'm guessing) some slight enhancements, and since the number is larger (31 v 30), the throat size is a little larger too, but not as big as the 34. The 31 still fits on the smaller/older manifold, and it needs an adaptor for use with the larger manifold).

Rob wrote - The normal VW carburettors have just one throat (venturi). This is always open (it's called a "constant venturi" carburettor), so there is a huge variation in the amount of vacuum at the venturi, depending on how open the butterfly is, and of course the engine speed. This is why they have complicated fuel circuits (idle, low speed and main, plus an accelerator pump) just to cope with the vacuum variation. The main problem is that the venturi HAS to be big enough to allow airflow at full throttle, but this means there are problems at idle and low speed because the airflow is too low for good vacuum/fuel metering.

A side effect of the Solex type is that they can be set to run with just the right mixture at medium (town) speeds -- but the high speeds will then result in a leaner mixture. Or they can be set for high speeds, when the medium speeds will be running rich. Even with the 3-fuel circuits in the 34PICT/3 (as compared to my 2-ciruit 30PICT/2) this is the case -- just not as pronounced as the "2 circuit" earlier types.

Another note from Rob - The two NEW commonly available carburettors for aircooled VWs these days are the Brosol or Bocar 34PICT/3, and the H30/31. The 30/31 has the same 24mm throat as the 31PICT and 30PICT, and the 34PICT/3 has the same 26mm throat as the older 34PICT/3s and 34PICT/4s.

Questions Specifically Regarding the 30 PICT/2

Carburettor -

Question - I have a simple question about the Solex 30 PICT/2 carburettor. What does the bypass screw do? I have screwed it all the way in while it is running and it makes a small difference in the idle but not much. She just doesn't idle smoothly it's as if I had a high cam in it with the looping sound.

Rob responded - The 30PICT/2 is one of the most reliable Solex carburettors. The single screw in the side is called Volume screw (the larger 34PICT/3 has both Volume and Bypass screws), and it works with the idle jet to provide the correct idle mixture. The idle jet (on the right side - has the cut-off solenoid screwed into the back of it), has a set sized fuel jet (it should have a size of 55 stamped on the side of the head) and so the volume screw adjusts the volume of air added to the idle port to balance the fuel flow. Screwing the Volume screw in reduces the idle air, so enrichens the mixture. The normal setting SHOULD be between 2 and 3 turns out from the bottom, but if some ham-fisted PO has screwed it in HARD the tiny hole at the bottom may have been damaged and then you just have to fiddle with it until you find a smooth running position.

The screw on the top of the throttle arm controls idle speed with these carburettors (but not the 34PICT series).

So you set the idle speed to 850rpm or so, then use the volume screw to get the fastest idle, then reset the idle speed using the throttle arm screw.

A note - The design of the H30/31 is almost identical to the 34PICT/3 - it's just a smaller size (the 30/31 main venturi is 24mm, the 34 has a 26mm venturi). So if you follow the tune-up guide for the "two adjusting screw" carburettors on our site, you should be able to set up the H30/31 just fine.

One thing though - the 30/31 carburettor is usually delivered with very lean jetting, and if you are using it on a 1300 or 1500 it may be OK, but if you have a 1600cc engine in your '67, then it will almost certainly run lean, and be difficult to tune. See the jetting suggestions above.

Specific Tuning Questions -

Question - When I floor suddenly (when traveling at low revs)

and normally in first the engine dies and then after about a second it comes to life and runs fine. Could this be fixed by adjusting idling?

Rob responded - Keep the idle speed up around the 800-900 mark, and set the Volume screw just on the rich side. Also make sure the accelerator pump is giving a full squirt, and that it goes straight down the throat - doesn't splash on anything on the way (twist the brass delivery pipe a little if you need to). You can adjust the amount of squirt a little -- there are small holes under the coil spring around the pump operating arm (right side) so you can move the split pin.

Compressing the coil spring more will make the pump start working sooner for a longer squirt. Doing all that will reduce the hesitation somewhat. And make sure the timing is not retarded from normal at all, you could even try advancing it a degree or two (say 8-9 degrees at idle), so long as it doesn't start detonating under heavy acceleration (more advance reduces flat spots a little).

Question continued - I think I have figured out why when traveling in third or fourth at low rpms when I suddenly floor it jips and shakes a bit, I figure I'm flooding the engine and it can't pick up at those speeds!

Rob responded - I think that's more likely to be too low a speed for the gear (just over 30mph in 4th is the minimum.) And the fractional LEAN hesitation would be part of the cause too. Have you checked and gapped the plugs lately? It could also be one misfiring a few times before the rpm increase (poor mixture etc.).

Question - I replaced the volume screw when I rebuilt the carburettor a few weeks ago… As far as the hole being reamed out a bit, I looked inside with a flashlight but it's hard to tell ...

Rob responded - The carburettor metal is fairly soft and it wouldn't take much pressure on the screw to force the bottom of the hole larger, but you couldn’t see it with a flashlight down that tiny hole anyway. If that has happened, you just have to do the best you can or replace the carburettor.

Question continued - The throttle shaft was damp from leaking fuel (not dripping but a little wet), so I guess it does have a

vacuum leak. I know of a shaft rebuilt kit but I was told that it might not solve the problem … I still can't explain why I have good idle and acceleration (could be smoother but...) when the volume screw does not work the way it should.

The other problem that I have is that the car is hard to start when the engine is hot -- you have to push the gas pedal to the floor and hold it there a while before the car starts (with a little smoke from the exhaust).

Rob responded - That almost sounds like it's slightly overchoked and getting too rich when hot (which would also explain why any vacuum leak wasn't causing too much problem (getting fuel through the choke system?). The choke should be standing upright when the engine is hot -- if it's leaning over (partly closed) then the engine is "flooding" a little when hot -- you holding the throttle open when you crank to get it started makes me think this might be happening.

The choke position is adjusted by loosening (not removing) the three screws on the retaining ring and rotating the choke canister under the ring. The are alignment marks on the ring and canister body so you can see how much it's moved.

Regarding Carburettor Gaskets -

Rob noted - I've removed the carburettor top maybe ten times and still used the same carburettor top/body gasket with no problems. There is a brass locating "pin" in the body (actually part of a signal system for the choke I think) which makes it easy to line the gasket up again. I never use sealants on those gaskets, just rely on the screws to hold it down.

 

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  Design by Erin

Carburetor

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Bendix-Technico (Stromberg) 1-barrel downdraft carburetor model BXUV-3, with nomenclature

A carburetor (American spelling), carburettor, or carburetter (Commonwealth spelling) is a device that blends air and fuel for an internal combustion engine. It is sometimes shortened to carb in North America and the United Kingdom.

Contents

[hide]

1 Word origin 2 History and development 3 Principles 4 Operation

o 4.1 Basics o 4.2 Off-idle circuit o 4.3 Main open-throttle circuit o 4.4 Power valve o 4.5 Accelerator pump o 4.6 Choke o 4.7 Other elements

5 Fuel supply

o 5.1 Float chamber 6 Multiple carburetor barrels 7 Carburetor adjustment 8 Catalytic carburetors 9 See also

o 9.1 Manufacturers 9.1.1 European 9.1.2 Non-European

o 9.2 Small engines o 9.3 Other o 9.4 Historic

10 Further reading o 10.1 General information o 10.2 Patents

10.2.1 American 10.2.2 Other

11 References o 11.1 Footnotes

12 External links

[edit] Word origin

The word carburetor comes from the French carbure meaning "carbide"[1]. Carburer means to combine with carbon. In fuel chemistry, the term has the more specific meaning of increasing the carbon (and therefore energy) content of a fuel by mixing it with a volatile hydrocarbon.

[edit] History and development

The first carburetors were surface carburetors where the volatility of the petrol was utilized. The Austrian automobile pioneer Siegfried Marcus invented the “rotating brush carburettor”. This was further improved by the Hungarian engineers János Csonka and Donát Bánki in 1893.[2]

Frederick William Lanchester of Birmingham, England, experimented with the wick carburetor in cars. In 1896, Frederick and his brother built the first gasoline driven car in England, a single cylinder 5 hp (3.7 kW) internal combustion engine with chain drive. Unhappy with the performance and power, they re-built the engine the next year into a two cylinder horizontally opposed version using his new wick carburetor design.

In 1885, Wilhelm Maybach and Gottlieb Daimler developed a carburetor for their engine based on the Atomizer nozzle.

Carburetors were the usual fuel delivery method for most U.S. made gasoline-fueled engines up until the late 1980s, when fuel injection became the preferred method of automotive fuel delivery. In the U.S. market, the last carbureted cars were:

1990 (General public) : Oldsmobile Custom Cruiser, Buick Estate Wagon 1991 (Police) : Ford Crown Victoria Police Interceptor with the 5.8 L (351 cu in) engine. 1991 (SUV) : Jeep Grand Wagoneer with the AMC 360 engine. 1994 (Light truck) : Isuzu [3]

Elsewhere, certain Lada cars used carburetors until 2006. A majority of motorcycles still use carburetors due to lower cost and throttle response problems with early injection setups, but as of 2005 many new models are now being introduced with fuel injection. Carburetors are still found in small engines[citation needed] and in older or specialized automobiles, such as those designed for stock car racing.

[edit] Principles

The carburetor works on Bernoulli's principle: the faster air moves, the lower its static pressure, and the higher its dynamic pressure. The throttle (accelerator) linkage does not directly control the flow of liquid fuel. Instead, it actuates carburetor mechanisms which meter the flow of air being pulled into the engine. The speed of this flow, and therefore its pressure, determines the amount of fuel drawn into the airstream.

When carburetors are used in aircraft with piston engines, special designs and features are needed to prevent fuel starvation during inverted flight. Later engines used an early form of fuel injection known as a pressure carburetor.

Most production carbureted (as opposed to fuel-injected) engines have a single carburetor and a matching intake manifolds that divides and transports the air fuel mixture to the intake valves, though some engines (like motorcycle engines) use multiple carburetors on split heads. Multiple carburetor engines were also common enhancements for modifying engines in America from the 1950s to mid-1960s, as well as during the following decade of high-performance American muscle cars fueling different chambers of the engine's intake manifold.

Older engines used updraft carburetors, where the air enters from below the carburetor and exits through the top. This had the advantage of never "flooding" the engine, as any liquid fuel droplets would fall out of the carburetor instead of into the intake manifold; it also lent itself to use of an oil bath air cleaner, where a pool of oil below a mesh element below the carburetor is sucked up into the mesh and the air is drawn through the oil-covered mesh; this was an effective system in a time when paper air filters did not exist.

Beginning in the late 1930s, downdraft carburetors were the most popular type for automotive use in the United States. In Europe, the sidedraft carburetors replaced downdraft as free space in the engine bay decreased and the use of the SU-type carburetor (and similar units from other manufacturers) increased. Some small propeller-driven aircraft engines still use the updraft carburetor design.

Outboard motor carburetors are typically sidedraft, because they must be stacked one on top of the other in order to feed the cylinders in a vertically-oriented cylinder block.

1979 Evinrude Type I marine sidedraft carburetor

The main disadvantage of basing a Carburetor's operation on Bernoulli's principle is that, being a fluid dynamic device, the pressure reduction in a venturi tends to be proportional to the square of the intake air speed. The fuel jets are much smaller and limited mainly by viscosity, so that the fuel flow tends to be proportional to the pressure difference. So jets sized for full power tend to starve the engine at lower speed and part throttle. Most commonly this has been corrected by using multiple jets. In SU and other movable jet carburetors, it was corrected by varying the jet size. For cold starting, a different principle was used, in multi-jet carburetors. A flow resisting valve called a choke, similar to the throttle valve, was placed upstream of the main jet to reduce the intake pressure and suck additional fuel out of the jets.

[edit] Operation

Fixed-venturi, in which the varying air velocity in the venturi alters the fuel flow; this architecture is employed in most carburetors found on cars.

Variable-venturi, in which the fuel jet opening is varied by the slide (which simultaneously alters air flow). In "constant depression" carburetors, this is done by a vacuum operated piston connected to a tapered needle which slides inside the fuel jet. A simpler version exists, most commonly found on small motorcycles and dirt bikes, where the slide and needle is directly controlled by the throttle position. The most common variable venturi (constant depression) type carburetor is the sidedraft SU carburetor and similar models from Hitachi, Zenith-Stromberg and other makers. The UK location of the SU and Zenith-Stromberg companies helped these carburetors rise to a position of domination in the UK car market, though such carburetors were also very widely used on Volvos and other non-UK makes. Other similar designs have been used on some European and a few Japanese automobiles. These carburetors are also referred to as "constant velocity" or "constant vacuum" carburetors. An interesting variation was Ford's VV (Variable Venturi) carburetor, which was essentially a fixed venturi carburetor with one side of the venturi hinged and movable to give a narrow throat at low rpm and a wider throat at high rpm. This was designed to provide good mixing and airflow over a range of engine speeds, though the VV carburetor proved problematic in service.

A high performance 4-barrel carburetor.

Under all engine operating conditions, the carburetor must:

Measure the airflow of the engine Deliver the correct amount of fuel to keep the fuel/air mixture in the proper range (adjusting for

factors such as temperature) Mix the two finely and evenly

This job would be simple if air and gasoline (petrol) were ideal fluids; in practice, however, their deviations from ideal behavior due to viscosity, fluid drag, inertia, etc. require a great deal of complexity to compensate for exceptionally high or low engine speeds. A carburetor must provide the proper fuel/air mixture across a wide range of ambient temperatures, atmospheric pressures, engine speeds and loads, and centrifugal forces:

Cold start Hot start Idling or slow-running Acceleration High speed / high power at full throttle Cruising at part throttle (light load)

In addition, modern carburetors are required to do this while maintaining low rates of exhaust emissions.

To function correctly under all these conditions, most carburetors contain a complex set of mechanisms to support several different operating modes, called circuits.

[edit] Basics

Cross Sectional schematic of a Carburetor

A carburetor basically consists of an open pipe through which the air passes into the inlet manifold of the engine. The pipe is in the form of a venturi: it narrows in section and then widens again, causing the airflow to increase in speed in the narrowest part. Below the venturi is a butterfly valve called the throttle valve — a rotating disc that can be turned end-on to the airflow, so as to hardly restrict the flow at all, or can be rotated so that it (almost) completely blocks the flow of air. This valve controls the flow of air through the carburetor throat and thus the quantity of air/fuel mixture the system will deliver, thereby regulating engine power and speed. The throttle is connected, usually through a cable or a mechanical linkage of rods and joints or rarely by pneumatic link, to the accelerator pedal on a car or the equivalent control on other vehicles or equipment.

Fuel is introduced into the air stream through small holes at the narrowest part of the venturi and at other places where pressure will be lowered when not running on full throttle. Fuel flow is adjusted by means of precisely-calibrated orifices, referred to as jets, in the fuel path.

[edit] Off-idle circuit

As the throttle is opened up slightly from the fully-closed position, the throttle plate uncovers additional fuel delivery holes behind the throttle plate where there is a low pressure area created by the throttle plate blocking air flow; these allow more fuel to flow as well as compensating for the reduced vacuum that occurs when the throttle is opened, thus smoothing the transition to metering fuel flow through the regular open throttle circuit.

[edit] Main open-throttle circuit

As the throttle is progressively opened, the manifold vacuum is lessened since there is less restriction on the airflow, reducing the flow through the idle and off-idle circuits. This is where the venturi shape of the carburetor throat comes into play, due to Bernoulli's principle (i.e., as the velocity increases, pressure falls). The venturi raises the air velocity, and this high speed

and thus low pressure sucks fuel into the airstream through a nozzle or nozzles located in the center of the venturi. Sometimes one or more additional booster venturis are placed coaxially within the primary venturi to increase the effect.

As the throttle is closed, the airflow through the venturi drops until the lowered pressure is insufficient to maintain this fuel flow, and the idle circuit takes over again, as described above.

Bernoulli's principle, which is a function of the velocity of the fluid, is a dominant effect for large openings and large flow rates, but since fluid flow at small scales and low speeds (low Reynolds number) is dominated by viscosity, Bernoulli's principle is ineffective at idle or slow running and in the very small carburetors of the smallest model engines. Small model engines have flow restrictions ahead of the jets to reduce the pressure enough to suck the fuel into the air flow. Similarly the idle and slow running jets of large carburetors are placed after the throttle valve where the pressure is reduced partly by viscous drag, rather than by Bernoulli's principle. The most common rich mixture device for starting cold engines was the choke, which works on the same principle.

[edit] Power valve

For open throttle operation a richer mixture will produce more power, prevent pre-ignition detonation, and keep the engine cooler. This is usually addressed with a spring-loaded "power valve", which is held shut by engine vacuum. As the throttle opens up, the vacuum decreases and the spring opens the valve to let more fuel into the main circuit. On two-stroke engines, the operation of the power valve is the reverse of normal — it is normally "on" and at a set rpm it is turned "off". It is activated at high rpm to extend the engine's rev range, capitalizing on a two-stroke's tendency to rev higher momentarily when the mixture is lean.

Alternative to employing a power valve, the carburetor may utilize a metering rod or step-up rod system to enrich the fuel mixture under high-demand conditions. Such systems were originated by Carter Carburetor[citation needed] in the 1950s for the primary two venturis of their four barrel carburetors, and step-up rods were widely used on most 1-, 2-, and 4-barrel Carter carburetors through the end of production in the 1980s. The step-up rods are tapered at the bottom end, which extends into the main metering jets. The tops of the rods are connected to a vacuum piston and/or a mechanical linkage which lifts the rods out of the main jets when the throttle is opened (mechanical linkage) and/or when manifold vacuum drops (vacuum piston). When the step-up rod is lowered into the main jet, it restricts the fuel flow. When the step-up rod is raised out of the jet, more fuel can flow through it. In this manner, the amount of fuel delivered is tailored to the transient demands of the engine. Some 4-barrel carburetors use metering rods only on the primary two venturis, but some use them on both primary and secondary circuits, as in the Rochester Quadrajet.

[edit] Accelerator pump

Liquid gasoline, being denser than air, is slower than air to react to a force applied to it. When the throttle is rapidly opened, airflow through the carburetor increases immediately, faster than the fuel flow rate can increase. This transient oversupply of air causes a lean mixture, which

makes the engine misfire (or "stumble")—an effect opposite what was demanded by opening the throttle. This is remedied by the use of a small piston or diaphragm pump which, when actuated by the throttle linkage, forces a small amount of gasoline through a jet into the carburetor throat.[4] This extra shot of fuel counteracts the transient lean condition on throttle tip-in. Most accelerator pumps are adjustable for volume and/or duration by some means. Eventually the seals around the moving parts of the pump wear such that pump output is reduced; this reduction of the accelerator pump shot causes stumbling under acceleration until the seals on the pump are renewed.

The accelerator pump is also used to prime the engine with fuel prior to a cold start. Excessive priming, like an improperly-adjusted choke, can cause flooding. This is when too much fuel and not enough air are present to support combustion. For this reason, most carburetors are equipped with an unloader mechanism: The accelerator is held at wide open throttle while the engine is cranked, the unloader holds the choke open and admits extra air, and eventually the excess fuel is cleared out and the engine starts.

[edit] Choke

When the engine is cold, fuel vaporizes less readily and tends to condense on the walls of the intake manifold, starving the cylinders of fuel and making the engine difficult to start; thus, a richer mixture (more fuel to air) is required to start and run the engine until it warms up. A richer mixture is also easier to ignite.

To provide the extra fuel, a choke is typically used; this is a device that restricts the flow of air at the entrance to the carburetor, before the venturi. With this restriction in place, extra vacuum is developed in the carburetor barrel, which pulls extra fuel through the main metering system to supplement the fuel being pulled from the idle and off-idle circuits. This provides the rich mixture required to sustain operation at low engine temperatures.

In addition, the choke can be connected to a cam (the fast idle cam) or other such device which prevents the throttle plate from closing fully while the choke is in operation. This causes the engine to idle at a higher speed. Fast idle serves as a way to help the engine warm up quickly, and give a more stable idle while cold by increasing airflow throughout the intake system which helps to better atomize the cold fuel.

In many carbureted cars, the choke is controlled by a cable connected to a pull-knob on the dashboard operated by the driver. In some carbureted cars it is automatically controlled by a thermostat employing a bimetallic spring, which is exposed to engine heat, or to an electric heating element. This heat may be transferred to the choke thermostat via simple convection, via engine coolant, or via air heated by the exhaust. More recent designs use the engine heat only indirectly: A sensor detects engine heat and varies electrical current to a small heating element, which acts upon the bimetallic spring to control its tension, thereby controlling the choke. A choke unloader is a linkage arrangement that forces the choke open against its spring when the vehicle's accelerator is moved to the end of its travel. This provision allows a "flooded" engine to be cleared out so that it will start.

Some carburetors do not have a choke but instead use a mixture enrichment circuit, or enrichener. Typically used on small engines, notably motorcycles, enricheners work by opening a secondary fuel circuit below the throttle valves. This circuit works exactly like the idle circuit, and when engaged it simply supplies extra fuel when the throttle is closed.

Classic British motorcycles, with side-draft slide throttle carburetors, used another type of "cold start device", called a "tickler". This is simply a spring-loaded rod that, when depressed, manually pushes the float down and allows excess fuel to fill the float bowl and flood the intake tract. If the "tickler" is held down too long it also floods the outside of the carburetor and the crankcase below, and is therefore a fire hazard.

[edit] Other elements

The interactions between each circuit may also be affected by various mechanical or air pressure connections and also by temperature sensitive and electrical components. These are introduced for reasons such as response, fuel efficiency or automobile emissions control. Various air bleeds (often chosen from a precisely calibrated range, similarly to the jets) allow air into various portions of the fuel passages to enhance fuel delivery and vaporization. Extra refinements may be included in the carburetor/manifold combination, such as some form of heating to aid fuel vaporization such as an early fuel evaporator.

[edit] Fuel supply

[edit] Float chamber

Holley "Visi-Flo" model #1904 carburetors from the 1950s, factory equipped with transparent glass bowls.

To ensure a ready mixture, the carburetor has a "float chamber" (or "bowl") that contains a quantity of fuel at near-atmospheric pressure, ready for use. This reservoir is constantly replenished with fuel supplied by a fuel pump. The correct fuel level in the bowl is maintained by means of a float controlling an inlet valve, in a manner very similar to that employed in a cistern (e.g. a toilet tank). As fuel is used up, the float drops, opening the inlet valve and admitting fuel. As the fuel level rises, the float rises and closes the inlet valve. The level of fuel maintained in the float bowl can usually be adjusted, whether by a setscrew or by something crude such as bending the arm to which the float is connected. This is usually a critical

adjustment, and the proper adjustment is indicated by lines inscribed into a window on the float bowl, or a measurement of how far the float hangs below the top of the carburetor when disassembled, or similar. Floats can be made of different materials, such as sheet brass soldered into a hollow shape, or of plastic; hollow floats can spring small leaks and plastic floats can eventually become porous and lose their flotation; in either case the float will fail to float, fuel level will be too high, and the engine will not run unless the float is replaced. The valve itself becomes worn on its sides by its motion in its "seat" and will eventually try to close at an angle, and thus fails to shut off the fuel completely; again, this will cause excessive fuel flow and poor engine operation. Conversely, as the fuel evaporates from the float bowl, it leaves sediment, residue, and varnishes behind, which clog the passages and can interfere with the float operation. This is particularly a problem in automobiles operated for only part of the year and left to stand with full float chambers for months at a time; commercial fuel stabilizer additives are available that reduce this problem.

Usually, special vent tubes allow air to escape from the chamber as it fills or enter as it empties, maintaining atmospheric pressure within the float chamber; these usually extend into the carburetor throat. Placement of these vent tubes can be somewhat critical to prevent fuel from sloshing out of them into the carburetor, and sometimes they are modified with longer tubing. Note that this leaves the fuel at atmospheric pressure, and therefore it cannot travel into a throat which has been pressurized by a supercharger mounted upstream; in such cases, the entire carburetor must be contained in an airtight pressurized box to operate. This is not necessary in installations where the carburetor is mounted upstream of the supercharger, which is for this reason the more frequent system. However, this results in the supercharger being filled with compressed fuel/air mixture, with a strong tendency to explode should the engine backfire; this type of explosion is frequently seen in drag races, which for safety reasons now incorporate pressure releasing blow-off plates on the intake manifold, breakaway bolts holding the supercharger to the manifold, and shrapnel-catching ballistic nylon blankets surrounding the superchargers.

If the engine must be operated in any orientation (for example a chain saw), a float chamber cannot work. Instead, a diaphragm chamber is used. A flexible diaphragm forms one side of the fuel chamber and is arranged so that as fuel is drawn out into the engine the diaphragm is forced inward by ambient air pressure. The diaphragm is connected to the needle valve and as it moves inward it opens the needle valve to admit more fuel, thus replenishing the fuel as it is consumed. As fuel is replenished the diaphragm moves out due to fuel pressure and a small spring, closing the needle valve. A balanced state is reached which creates a steady fuel reservoir level, which remains constant in any orientation.

[edit] Multiple carburetor barrels

Holley model #2280 2-barrel carburetor

Colombo Type 125 "Testa Rossa" engine in a 1961 Ferrari 250TR Spider with six Weber two-barrel carburetors inducting air through 12 air horns; one individually adjustable barrel for each cylinder.

While basic carburetors have only one venturi, many carburetors have more than one venturi, or "barrel". Two barrel and four barrel configurations are commonly used to accommodate the higher air flow rate with large engine displacement. Multi-barrel carburetors can have non-identical primary and secondary barrel(s) of different sizes and calibrated to deliver different air/fuel mixtures; they can be actuated by the linkage or by engine vacuum in "progressive" fashion, so that the secondary barrels do not begin to open until the primaries are almost completely open. This is a desirable characteristic which maximizes airflow through the primary barrel(s) at most engine speeds, thereby maximizing the pressure "signal" from the venturis, but reduces the restriction in airflow at high speeds by adding cross-sectional area for greater airflow. These advantages may not be important in high-performance applications where part throttle operation is irrelevant, and the primaries and secondaries may all open at once, for simplicity and reliability; also, V-configuration engines, with two cylinder banks fed by a single carburetor, may be configured with two identical barrels, each supplying one cylinder bank. In the widely seen V8 and 4-barrel carburetor combination, there are often two primary and two secondary barrels.

The spread-bore 4-barrel carburetor, first released by Rochester in the 1965 model year as the "Quadrajet"[citation needed] has a much greater spread between the sizes of the primary and secondary throttle bores. The primaries in such a carburetor are quite small relative to conventional 4-barrel practice, while the secondaries are quite large. The small primaries aid low-speed fuel economy and drivability, while the large secondaries permit maximum performance when it is called for. To tailor airflow through the secondary venturis, each of the secondary throats has an air valve at the top. This is configured much like a choke plate, and is lightly spring-loaded into the closed position. The air valve opens progressively in response to engine speed and throttle opening, gradually allowing more air to flow through the secondary side of the carburetor. Typically, the air valve is linked to metering rods which are raised as the air valve opens, thereby adjusting secondary fuel flow.

Multiple carburetors can be mounted on a single engine, often with progressive linkages; two four-barrel carburetors were frequently seen on high performance American V8s, and multiple two barrel carburetors are often now seen on very high performance engines. Large numbers of small carburetors have also been used (see photo), though this configuration can limit the maximum air flow through the engine due to the lack of a common plenum; with individual intake tracts, not all cylinders are drawing air at once as the engine's crankshaft rotates[5].

[edit] Carburetor adjustment

Too much fuel in the fuel-air mixture is referred to as too rich, and not enough fuel is too lean. The mixture is normally adjusted by one or more needle valves on an automotive carburetor, or a pilot-operated lever on piston-engined aircraft (since mixture is air density (altitude) dependent). The (stoichiometric) air to gasoline ratio is 14.7:1, meaning that for each weight unit of gasoline, 14.7 units of air will be consumed. Stoichiometric mixture are different for various fuels other than gasoline.

Ways to check carburetor mixture adjustment include: measuring the carbon monoxide, hydrocarbon, and oxygen content of the exhaust using a gas analyzer, or directly viewing the colour of the flame in the combustion chamber through a special glass-bodied spark plug sold under the name "Colortune"[6] for this purpose. The flame colour of stoichiometric burning is described as a "bunsen blue", turning to yellow if the mixture is rich and whitish-blue if too lean.

The mixture can also be judged after engine running by the state and color of the spark plugs: black, dry sooty plugs indicate a too rich mixture, white to light gray deposits on the plugs indicate a lean mixture. The correct color should be a brownish gray. See also reading spark plugs.

In the early 1980s, many American-market vehicles used special "feedback" carburetors that could change the base mixture in response to signals from an exhaust gas oxygen sensor. These were mainly used to save costs (since they worked well enough to meet 1980s emissions requirements and were based on existing carburetor designs), but eventually disappeared as falling hardware prices and tighter emissions standards made fuel injection a standard item.

Where multiple carburetors are used the mechanical linkage of their throttles must be synchronized for smooth engine running.

[edit] Catalytic carburetors

A catalytic carburetor mixes fuel fumes with water and air in the presence of heated catalysts such as nickel or platinum. This breaks the fuel down into methane, alcohols, and other lighter-weight fuels. The original catalytic carburetor was introduced to permit farmers to run tractors from modified and enriched kerosene. The U.S. Army also used catalytic carburetors with great success in World War II, in the North African desert campaign[citation needed].

While catalytic carburetors were made commercially available in the early 1930s, two major factors limited their widespread public use. First, the addition of additives to commercial gasoline made it unsuitable for use in engines with catalytic carburetors. (Tetra-ethyl lead, for example, was introduced in 1932 to raise gasoline's resistance to engine knock, thereby permitting the use of higher compression ratios.) Second, the economic advantage of using kerosene over gasoline faded in the 1930s, eliminating the catalytic carburetor's primary advantage.

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