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CRAZY HORSE MOTORCYCLESCarburetor Class


Mikuni Motorcycle Carburetor Theory

Motorcycle carburetors look very complex, but with a little theory, you can tune your bike for maximum performance.

All carburetors work under the basic principle of atmospheric pressure. Atmospheric pressure is a powerful force which exerts pressure on everything. It varies slightly but is generally considered to be 15 pounds per square inch (PSI).

This means that atmospheric pressure is pressing on everything at 15 PSI. By varying the atmospheric pressure inside the engine and carburetor, we can change the pressure and make fuel and air flow.

Atmospheric pressure will force high pressure to low pressure. As the piston goes down on a four stroke engine a low pressure is formed above the piston on a four stroke.

This low pressure also causes a low pressure inside the carburetor. Since the pressure is higher outside the engine and carburetor, air will rush inside the carburetor and engine until the pressure is equalized.

The moving air going through the carburetor will pick up fuel and mix with the air.


Cycles


Engine layout


• Inside a carburetor is a Venturi. The Venturi is a restriction inside the carburetor that forces air to speed up to get through. A river that suddenly narrows can be used to illustrate what happens inside a carb. The water in the river speeds up as it gets near the narrowed shores and will get faster if the river narrows even more. The same thing happens inside the carburetor. The air that is speeding up will cause atmospheric pressure to drop inside the carburetor. The faster the air moves, the lower the pressure inside the carburetor.


Carburetor CircuitsMost motorcycle carburetor circuits are governed by throttle position and not by engine speed.

There are five main metering systems inside most motorcycle carburetors. These metering circuits overlap each other and they are:

* pilot circuit* throttle valve* needle jet and jet needle* main jet* choke circuit


Drawing of Carb


View of Carb


Pilot Circuit The pilot circuit has two adjustable parts. The pilot air screw and pilot jet. The air screw can be located either near the back side of the carburetor or near the front of the carburetor.

If the screw is located near the back, it regulates how much air enters the circuit. If the screw is turned in, it reduces the amount of air and richens the mixture.

If it is turned out, it opens the passage more and allows more air into the circuit which results in a lean mixture. If the screw is located near the front, it regulates fuel.

The mixture will be leaner if it is screwed in and richer if screwed out. If the air screw has to be turned more than 2 turns out for best idling, the next smaller size pilot jet will be needed.

The pilot jet is the part which supplies most of the fuel at low throttle openings. It has a small hole in it which restricts fuel flow though it. Both the pilot air screw and pilot jet affects carburetion from idle to around 1/4 throttle.


Slide ValveThe slide valve affects carburetion between 1/8 thru 1/2 throttle. It especially affects it between 1/8 and 1/4 and has a lesser affect up to 1/2. The slides come in various sizes and the size is determined by how much is cutaway from the backside of it, fig 3. The larger the cutaway, the leaner the mixture (since more air is allowed through it) and the smaller the cutaway, the richer the mixture will be. Throttle valves have numbers on them that explains how much the cutaway is. If there is a 3 stamped into the slide, it has a 3.0mm cutaway, while a 1 will have a 1.0mm cutaway

(which will be richer than a 3).


Jet NeedleThe jet needle and needle jet affects carburetion from 1/4 thru 3/4 throttle. The jet needle is a long tapered rod that controls how much fuel can be drawn into the carburetor venturi.

The thinner the taper, the richer the mixture. The thicker the taper, the leaner the mixture since the thicker taper will not allow as much fuel into the venturi as a leaner one. The tapers are designed very precisely to give different mixtures at different throttle openings. Jet needles have grooves cut into the top.

A clip goes into one of these grooves and holds it from falling or moving from the slide. The clip position can be changed to make an engine run richer or leaner, fig 4. If the engine needs to run leaner, the clip would be moved higher. This will drop the needle farther down into the needle jet and cause less fuel to flow past it. If the clip is lowered, the jet needle is raised and the mixture will be richer.

The needle jet is where the jet needle slides into. Depending on the inside diameter of the needle jet, it will affect the jet needle. The needle jet and jet needle work together to control the fuel flow between the 1/8 thru 3/4 range. Most of the tuning for this range is done to the jet needle, and not the needle jet.


Jet Needle


Main Jet

Once the throttle is opened far enough, the jet needle is pulled high enough out of the needle jet and the size of the hole in the main jet begins to regulate fuel flow. Main jets have different size holes in them and the bigger the hole, the more fuel that will flow (and the richer the mixture). The higher the number on the main jet, the more fuel that can flow through it and the richer the mixture.

The main jet controls fuel flow from 3/4 thru full throttle.


ChokeThe choke system is used to start cold engines.

Since the fuel in a cold engine is sticking to the cylinder walls due to condensation, the mixture is too lean for the engine to start.

The choke system will add fuel to the engine to compensate for the fuel that is stuck to the cylinder walls.

Once the engine is warmed up, condensation is not a problem, and the choke is not needed.


Air/Fuel Mixture

The air/fuel mixture must be changes to meet the demands of the needs of the engine. The ideal air/fuel ratio is 14.7 grams of air to 1 gram of fuel. This ideal ratio is only achieved for a very short period while the engine is running. Due to the incomplete vaporization of fuel at slow speeds or the additional fuel required at high speeds, the actual operational air/fuel ratio is usually richer.

The drawing to the right shows the actual air/fuel ratio for any given throttle opening.


Working Range


Jetting Troubleshooting

Carburetor troubleshooting is simple once the basic principles are known.

The first step is to find where the engine is running poorly.

It must be remembered that carburetor jetting is determined by the throttle position, not engine speed.

If the engine is having troubles at low rpm (idle to 1/4 throttle), the pilot system or slide valve is the likely problem.

If the engine has problems between 1/4 and 3/4 throttle, the jet needle and needle jet (most likely the jet needle) is likely the problem.

If the engine is running poorly at 3/4 to full throttle, the main jet is the likely problem.


Preparing to TuneWhile jetting carburetors, place a piece of tape on the throttle housing. Place another piece of tape on the throttle grip and draw a line (while the throttle is at idle) straight across from one piece of tape to the other. When these two lines are lined up, the engine will be idling.

Now open the throttle to full throttle and draw another line directly across from it on the throttle housing. At this point, there should be two lines on the throttle housing, and one on the throttle grip. Now find the half-way point between both of the lines on the throttle housing.

Make a mark and this will show when the throttle is at half throttle. Divide the spaces up even again until idle, 1/4, 1/2, 3/4, and full throttle positions are known. These lines will be used to quickly find the exact throttle opening while jetting.


Start Tuning Clean the air filter and warm the bike up. Accelerate through the gears until the throttle is at full throttle (a slight uphill is the best place for this).

After a few seconds of full throttle running, quickly pull in the clutch and stop the engine (Do not allow the engine to idle or coast to a stop). Remove the spark plug and look at its color. It should be a light tan color (for more info on reading spark plugs click here). If it's white, the air/fuel mixture is too lean and a bigger main jet will have to be installed. If it's black or dark brown, the air/fuel mixture is too rich and a smaller main jet will have to be installed. While changing jets, change them one size at a time, test run after each change, and look at the plug color after each run.

After the main jet has been set, run the bike at half throttle and check the plug color. If it's white, lower the clip on the jet needle to richen the air/fuel mixture. If it's dark brown or black, raise the clip to lean the air/fuel mixture.

The pilot circuit can be adjusted while the bike is idling and then test run. If the engine is running poorly just off of idle, the pilot jet screw can be turned in or out to change the air-fuel mixture. If the screw is in the back of the carburetor, screwing it out will lean the mixture while screwing it in will richen it.

If the adjustment screw is in the front of the carburetor, it will be the opposite. If turning the screw between one and two and a half doesn't have any affect, the pilot jet will have to be replaced with either a larger or smaller one. While adjusting the pilot screw, turn it 1/4 turn at a time and test run the bike between adjustments. Adjust the pilot circuit until the motorcycle runs cleanly off of idle with no hesitations or bogs.


Altitude, Humidly, and Air Temperature

Once the jetting is set and the bike is running good, there are many factors that will change the performance of the engine.

Altitude, air temperature, and humidity are big factors that will affect how an engine will run. Air density increases as air gets colder. This means that there are more oxygen molecules in the same space when the air is cold.

When the temperature drops, the engine will run leaner and more fuel will have to be added to compensate. When the air temperature gets warmer, the engine will run richer and less fuel will be needed. An engine that is jetted at 32deg Fahrenheit may run poorly when the temperature reaches 90deg Fahrenheit.

Altitude affects jetting since there are less air molecules as altitude increases.

A bike that runs good at sea level will run rich at 10,000 ft due to the thinner air.


Correction factors Humidity is how much moister is in the air. As humidity increases, jetting will be richer. A bike that runs fins in the mornings dry air may run rich as the day goes on and the humidity increases.

Correction factors are sometimes used to find the correct carburetor settings for changing temperatures and altitudes.

The chart in fig 8, shows a typical correction factor chart. To use this chart, jet the carburetor and write down the pilot and main jet sizes. Determine the correct air temperature and follow the chart over to the right until the correct elevation is found. Move straight down from this point until the correct correction factor is found.

Using fig 8 as an example, the air temperature is 95deg Fahrenheit and the altitude is 3200 ft. The correction factor will be 0.92. To find out the correction main and pilot jets, multiple the correction factor and each jet size. A main jet size of 350 would be multiplied by 0.92 and the new main jet size would be a 322. A pilot jet size of 40 would be multiplied by 0.92 and the pilot jet size would be 36.8.


Correction factors


Correction factorsCorrection factors can also be used to find the correct settings for the needle jet, jet needle, and air screw. Use the chart and determine the correction factor. Then use the table below to determine what to do with the needle jet, jet needle, and air screw.


Roll Off Method1: Main Jet Size: How to Get it Right

Mikuni HSR-series carburetors are remarkably versatile instruments. The standard tuning seldom needs more than small adjustments to accommodate a wide range of engine set-ups. One of the more common required changes is the main jet size.

Aftermarket exhausts have a wide range of flow volumes and the best main jet size is closely associated with exhaust flow. Thus, it is often necessary to replace the standard main jet with a different size to accommodate the wide range of exhaust designs on the market. However, it is easy to get the main jet right for a particular exhaust system using one of the techniques described on this page.

The standard main jet fitted to the HSR42 is a number 160. This size is correct for stock mufflers. Typically, an HSR42 combined with aftermarket exhaust system needs a 165 main jet. The general rule is that HSR42s fitted to engines with loud exhausts usually run best with a 165 main jet.

The HSR45 has a number 175 and the HSR48 a 190. These jets are more suited to modified engines with free flowing exhaust systems.


Roll Off MethodKeep in mind that the main jet does not affect mixtures until approximately 3/4 throttle. Below that throttle setting, specifically between 1/4 and 3/4 throttle, air/fuel mixtures are controlled by the jet needle and needle jet.

It is relatively easy to get the main jet correct. Follow either of the techniques described below. Both are satisfactory but the Roll-On procedure is more accurate.

NOTE:

The following tuning techniques might result in excessive (illegal) speed and increased risk from the speed and the necessary distraction of doing the test. We recommend that the testing be done on a closed course (track) or on a dynamometer, if one is available.


Roll Off MethodROLL-OFF:

The Roll-Off technique is the quickest and is almost as accurate as the Roll-On method. First, one gets the engine warm on the way to a safe roadway. If there is room, use fourth gear as this allows more time to assess the result.

Now, get the engine rpm high enough that it is on the cam and in its power band. This may need to be as high as 4000 rpm with some cam choices. Apply full throttle. Let the engine accelerate for a couple of seconds until it has settled in and is pulling hard. Quickly roll the throttle off to about the 7/8ths position. When you do this, the mixture richens slightly for a second or so.If the engine gains power as you roll the throttle off, then the main jet is too small and you need to fit a larger one. If the engine staggers slightly or has a hard hesitation, then the main jet is too large and you need to fit a smaller one.


Poor Mid-Range Performance2: Poor Mid-Range Performance

Possible Causes:

1.Carburetor Tuning2.Exhaust system3.Too much cam4.Ignition5.Low compression pressure

Carburetor Tuning:

Typically, mid-range performance is controlled by the jet needle/needle jet combination. This is because the majority of mid-rpm operation is at low throttle settings or on the highway at cruising speeds of 50 - 70 mph. The HSR42 or HSR45 can deliver enough air/fuel mixture to support these speeds with throttle openings between 1/8th & 1/4, where the straight-diameter part of the jet needle controls fuel flow.


Mid-Ranged PerformanceMikuni supplies four different jet needle sizes to accommodate tuning requirements in this range, one set of four for the HSR42, four for the HSR45 and another set for the HSR48. They differ only in the diameter of the straight section of the needle. The leanest is J8-8DDY01-98 (HSR42 example part number) and the richest is J8-8CFY02-95 (HSR45 example part number). We commonly refer to these needles by their "dash" number (-95, -96, -97 or -98).

Flat throttle response in the mid-rpm range is seldom caused by either an over-rich or overly lean condition. Flat mid-rpm performance is more likely due to the effects of the cam or exhaust design. If the needle size is incorrect, it will normally reveal itself as poor mileage (too rich), slow warm-up (too lean) or light detonation when accelerating moderately from around 2500 to 2900 rpm (again, too lean).

A typical FXD (either engine type) motorcycle will deliver around 45 mpg at 65 mph on a flat, windless road. A heavy touring machine (FLHT- series) may be down a few mpg from that standard. Fuel mileage in the 30s indicates a rich condition.


Mid-Range PerformancePlease refer to the tuning manual, available on the Manuals page for instructions on diagnosing and tuning.

Note:

Confusing symptoms is one of the most common errors in diagnosing carburetor tuning inaccuracies. For instance, low power at 60 mph (2500 rpm) in top gear may have one or more of several causes: The exhaust system may not work well at that rpm, the cam design may not work well at that rpm, the ignition timing could be incorrect for that rpm, or, --- the carburetor could be set too lean or too rich at that throttle opening.

Notice that when the carburetor was mentioned above, it is the throttle opening we refer to and not the rpm. This is an important difference.

While the performance of other engine components depend, to a large extent, upon rpm, the carburetor only responds to the position of its throttle valve (slide) and the amount of air flowing through it (and sometimes the direction of that air flow).


Mid-Range

One of the most valuable carburetor tuning aids is to change rpm (down or up shift) while holding the same road speed. An example: The engine gives poor acceleration from 60 mph (2570 rpm) in top gear. If you maintain the road speed and down shift to fourth gear, the throttle setting will remain essentially the same but the engine rpm will increase 20%. If the poor top gear acceleration is due to, say, poor exhaust system performance at that rpm, then, the problem will either go away, get better or at least change its character. If, on the other hand, the problem is carburetor tuning, the poor acceleration will remain the same because the carburetor throttle opening is the same.

Exhaust system:

Straight pipes:

Open straight pipes perform poorly in the 2500 to 3800 rpm range. If they are 34" or longer, they do not perform really well at any rpm.


Mid-RangeSymptoms include missing, backfiring through the carburetor, reversion (fuel dripping out of the air cleaner) and poor acceleration.

Open mufflers:

"Gutted" mufflers with stock (or stock-like) header pipes tend to perform poorly in the same rpm range as straight pipes and exhibit similar symptoms.Long thin mufflers:

Long, small diameter mufflers with full-length baffles often exhibit the same symptoms as straight pipes, although their over-all performance may be better.High performance 2-into-1 systems:

These systems are often poor performers in the 2000 to 3000 rpm range. Most 2-into-1 exhaust systems deliver a significant torque dip at 2500 which is slightly less than 60 mph in top gear for most stock Harley Big Twins.Header pipe diameter:


Mid-RangeThe great majority of Harley engines, of any displacement, do their best work with 1-3/4" diameter exhaust pipes. Larger pipes tend to suppress mid-rpm performance and, for that matter, seldom deliver the best power at high rpm either.

Header pipe length:

The stock header pipe is about 30". Multiple tests, made by several groups, confirm this length as being very nearly the best for all-round performance. Shorter (less than 27") and longer (over 32") header pipes significantly reduce peak power, throttle response and over-all performance. An exception to this "rule" are a couple of the high performance 2-into-1 systems which work very well with longer (and un-even) header pipe lengths. Stock Harley header pipes are near-perfect in diameter and length.Muffler size:

It is not possible to make a muffler quiet, small and powerful at the same time. One can choose power and small, quiet and small but not all three. The reason stock mufflers are poor performers is because they are small and quiet.


Mid-RangeHowever, small and loud is not a guarantee of performance. In general, small mufflers with large straight-through, perforated tube baffles (looks like a tube with many holes drilled in it) make the most power and the most noise. An exception to this rule (there may be more) are the popular H-D Screamin' Eagle (and Cycle Shack) small slip-on mufflers which perform very well yet are not straight-through designs. The popular louvered core baffles restrict flow at full throttle & high rpm and reduce power a bit as a result.

Too much cam:

The most important cam timing event is when the intake valve closes. The intake closing point determines the minimum rpm at which the engine begins to do its best work. The later the intake valves close, the higher the rpm must be before the engine gets "happy." High rpm cam designs often perform poorly in the rpm range associated with ordinary riding. The problem with such choices is that the engine seldom spends time in the rpm range favored by such cams. Unfortunately, in the quest for maximum power output, many-too-many Harley owners choose a late-closing, high-rpm cam for their engine.


Mid-RangeA majority of any Harley motor's life is spent in the mid-portion of is rpm limits, between 2000 and 4000 rpm. At open-road cruising speeds, that range is more like 2500 to 3500 rpm. With current Big Twin gearing, top gear at 2500 rpm returns a road speed of 60 mph and 3500 delivers 84 mph.

Riders sometimes "putt" around at 2000 or less. Even when accelerating to cruising speed, few of us use more than 4000 - 4500 rpm as a shift point. Very seldom, in day-to-day use, do our engines get near 5000 rpm, let alone 6000.Even the mildest of Harley-Davidson's aftermarket cams (Evo or Twin Cam) do their best work above 3000 rpm. At 2000, the majority these cams seldom perform as well as the stock cam(s).

The rpm at which a Big Twin gets "happy" can be predicted by the closing point (angle) of the intake valves. The angle is expressed as the number of degrees After Bottom Dead Center (ABDC) that the valves reach .053" from being fully seated.

30 degrees = 2400 rpm35 degrees = 3000 rpm40 degrees = 3600 rpm45 degrees = 4000 rpm50+ degrees = 4500 rpm


Mid-RangeThese relationships are approximate but should hold true to within 200 rpm or so. They also assume that all other tuning factors, exhaust, ignition, etc., are operating correctly.

If you have one of the late-closing cam designs installed, say one that closes the intake valves later than 40 degrees, then you cannot expect excellent performance at 2000 rpm. No carburetor adjustment, ignition adjustment or exhaust system can change this.

Ignition:

Stock H-D Evo Big Twin ignitions have two advance curves ---- a quick advance curve for part-throttle, light load running, and, the very slow advance curve for mid to full-throttle running. It is this second curve that determines the ignition timing when accelerating even moderately.

While not the most common reason for 'soft' or 'flat' acceleration in the mid-rpm range, the stock Evo ignition doesn't help.

The Screamin' Eagle Evo ignitions have the same full throttle advance curve as the stock ignition. The only difference between the two is the rev limiter rpm which is 5200 for the stock unit and 8000 (much too high) for the Screamin' Eagle ignition.


Mid-RangeIgnitions with quicker advance curves, such as the CompuFire (curves 6,7 or 8) or Dyna 2000 (#1 curve only) have aggressive advance curves and improve throttle response and part-throttle performance in the mid-rpm range, especially below 3000 rpm.

These two examples are that only; there are other after market ignitions that also contain quicker advance curves.

Stock Twin Cam ignitions are more complex than the earlier Evo type. They use a manifold pressure/engine revolution rate system for choosing ignition timing for any combination of rpm and throttle setting. We have no reason to recommend non-Harley ignitions for the Twin Cam engines.

Low compression pressure:

The higher the pressure within the combustion chamber when the air/fuel mixture is ignited, everything else being equal, the more power your engine produces and more efficiently it runs. However, if the pressure it too high, detonation (pinging) may occur which can destroy an engine.

Each combustion chamber design has an upper pressure limit above which serious, damaging detonation is likely. With modern American 92 Octane lead-free gasoline, a reasonable upper pressure limit is 180 psi for the Evo Big Twin and 190 psi for the Twin Cam. A well-tuned motor should not suffer detonation with these pressures.


Mid-Range

The standard method for determining the compression or cranking pressure of an engine is to remove the spark plugs, install a standard compression gauge into one of the spark plug holes and, with the throttle full-open, crank the engine over with the starter motor until the pressure gauge needle stops rising. This usually takes 4 - 8 compression strokes. Both cylinders should be tested.

Stock Evo and Twin Cam motors develop cranking pressures in the 150 psi range. If a late-closing cam is installed, with no other changes, the cranking pressure will go down. The reason high compression ratio pistons and racing cams are so often associated is because the higher compression ratio pistons (and/or milled heads) are needed to regain even the normal moderate cranking pressures, let alone raise them for more power and efficiency.

Low cranking pressures (because of late closing cams and stock pistons) can significantly reduce performance in the mid-rpm range.


BackfireBackfires Through Carburetor Common Causes:

Ignition: The factory Evolution engine's ignition can contribute backfiring through the carburetor.

Cam design: Long duration cams with early opening intake valves can contribute to backfiring.

Intake manifold air leak: A lean condition due to an intake manifold air leak can cause backfiring.

Carburetor jetting: An overly-lean low-speed circuit, non-functioning accelerator pump or clogged pilot jet can contribute to backfiring.


Backfire

Ignition:

Harley ignition systems have been “dual fire” for decades. Virtually all stock Evolution engines, Big Twin & Sportster, have dual fire ignitions. The PP100 used in the Gilroy era Indians came stock with duel fire. The exceptions are the EFI touring bikes and the 98 & later Sportster Sport models. All Twin Cam engines are fitted with single fire ignitions. Under normal conditions dual fire ignitions present no problems. However, when combined with high performance long duration cams the stock ignition can cause premature ignition of an air/fuel mixture entering the rear cylinder. This, in turn, results in backfiring through the open intake valve into the intake system.


BackfireDual fire ignitions fire front and rear cylinder spark plugs together. One of the sparks starts combustion while the other is wasted in other cylinder which is not on its firing stroke.

When the rear cylinder is getting a useful spark, the front cylinder ís spark is occurring near the middle of its exhaust stroke. There is nothing to burn in the front cylinder at this time.

However, when the front cylinder is getting its useful spark, the rear cylinder is on its intake stroke and a combustible mixture may be present. If that mixture is ignited by the “wasted” spark, then a backfire occurs as the burning mixture forces its way past the intake valve and out through the intake manifold and carburetor.

Single fire ignitions can often eliminate carburetor backfiring since they do not produce a wasted spark in the rear cylinder. In fact, single fire ignitions can generally eliminate backfiring in any Harley. For instance, EFI and Twin Cam engines very seldom backfire through their intakes; both have single fire ignition systems.


BackfireCam design:

The earlier the intake valve opens the more likely the dual fire ignition will ignite air/fuel mixture in the rear cylinder. High performance long duration cams open the intake valves earlier than the stock one. This is the main reason why modified Harley engines tend to backfire through the carburetor more frequently than stock engines.

Intake manifold air leak:

A common and continuing problem with Harley engines is air leaks around the junction of the manifold and the cylinder heads. Carburetor/manifold leaks are much less common. An air leak can cause carburetor backfiring.

Other symptoms of an air leak include a slow return to idle or an irregular idle.


BackfireCarburetor jetting:

Excessively lean carburetor settings can contribute to backfiring. If the mixture is too lean, it may burn very slowly and unevenly. This condition, in turn, may result in burning mixture remaining in the cylinder until the beginning of the next intake stroke when it can ignite the incoming air/fuel mixture.

A too-small or partially blocked pilot jet can bring about this condition.

An accelerator pump adjustment that starts the pump too late can cause this problem.

A partial vacuum in the fuel tank can reduce fuel flow and bring about a lean condition. The common factory Harley gas cap that incorporates a one-way valve (for emission purposes) sometimes restricts air flow into the tank. This restriction can result in a partial vacuum and fuel flow restriction.


Backfire through ExhaustBackfires in Exhaust Note:

It is normal for many high performance exhaust systems to moderately backfire or pop when the throttle is closed from mid-to-high rpm. In fact, one should expect a well-tuned high performance engine to "pop" and "crackle" when the throttle is closed at high rpm.

The popping is a result of the air/fuel mixture becoming very lean when the throttle is closed and the engine is rotating well above idle speed. It is also necessary that the exhaust system have rather open mufflers.


Backfire-ExhaustWhy This (normally) Happens:

1) When the throttle valve is in the idle position, fuel does not flow out of the main system (needle, needle jet, main jet). Fuel is only delivered to the engine by the pilot (idle) system.2)The combined effect of the closed throttle and elevated engine rpm is to create a fairly strong vacuum in the intake manifold. This vacuum, in turn, causes a high air flow rate through the small gap formed by the throttle valve and carburetor throat.3)Under these conditions the pilot (idle) system cannot deliver enough fuel to create a normal, combustible air/fuel ratio. The mixture becomes too lean to burn reliably in the combustion chamber. It gets sent into the exhaust system unburned and collects there.4)When the odd firing of the lean mixture does occur, it is sent, still burning, into the exhaust system where it sometimes ignites the raw mixture that has collected ---- the exhaust then pops or backfires.5)Completely stock Harleys do not do this until open-end mufflers, such as the popular Screamin' Eagle slip-ons, are installed. The exhaust must be both free-flowing and have an open exit for the popping to occur.


Backfire-ExhaustOther possible causes:

Air Leaks:

Any source of fresh air into the exhaust system can create or worsen the conditions that bring about exhaust backfiring. The most common entry point is the junction of the header pipes and mufflers. Even a small air leak can dramatically increase the intensity or likelihood of exhaust system backfiring.

A high temperature silicone sealant, as can be found in many auto parts stores, may be used to seal the pipe/muffler junction. Lean Carburetion:

While exhaust system popping may be considered normal, it is certainly made worse by an overly lean idle circuit.


Backfire-Exhaust

Be sure that your carburetor's pilot jet is the correct size and that the idle air mixture screw is correctly adjusted before looking for other causes of popping. The procedure for adjusting the pilot circuit is covered in the Tuning Manual.Ignition:

If exhaust system popping is very loud, irregular and accompanied by loss of power, then you should suspect that the ignition system is not performing as it should. If, for some reason, the ignition sometimes fires at the wrong time, then exhaust popping can become very energetic (loud). Look for failing high tension leads (plug wires), failing ignition coil(s) and especially switches or connectors as possible causes.


Spark KnockDetonation ("Spark Knock")

Detonation, often called pinging, is nothing less than a series of small explosions that take place within an engine's combustion chambers. It can be extremely destructive, breaking pistons, rod bearings and anything else from the pistons down that a large hammer could damage. It is best avoided.

Pinging is a descriptive name for detonation. Pinging is that high pitch ringing sound that an engine sometimes makes when the throttle is opened with the engine under load. It sounds as though the cooling fins are ringing as they do when you quickly run your finger nail over their edges.

Pinging indicates trouble. Trouble that does damage. That damage can be quick and catastrophic but usually isn't. Most often, detonation occurrences are small in energy and the engine is able to absorb the punishment, at least temporarily. However, over time, even light detonation does harm; weakening pistons and overheating the top piston rings.

Severe detonation can destroy an engine literally in a heart beat.


Spark KnockHOW IT HAPPENS

After a spark ignites the air/fuel mixture in an engine's combustion chamber, the flame front travels across the chamber at a rate of about 5000 feet per second. That's right, one mile per second.

Flame front travel for detonation is closer to 19,000 to 25,000 feet per second; the same rate as in dynamite. The difference between normal combustion and detonation is the rate at which the burning takes place and therefore the rate of pressure rise in the chamber. The hammer like blows of detonation literally ring the metal structures of the motor and that is what you hear as pinging.

Detonation occurs when the air/fuel mixture ignites before it should. Normal burning has the flame front traveling from the spark plug(s) across the chamber in a predictable way. Peak chamber pressure occurs at about 12 degrees after top dead center and the piston gets pushed down the bore.


Spark Knock

Sometimes and for various reasons a second flame front starts across the chamber from the original source of ignition. The chamber pressure then rises too rapidly for piston movement to relieve it. The pressure and temperature become so great that all the mixture in the chamber explodes. If the force of that explosion is great enough --- the engine breaks.


Spark KnockWHAT CAUSES IT

Anytime the combustion chamber pressures become high enough, detonation occurs. Anything that creates such pressure is the cause of detonation.

Here is a list of possible causes, it may not be complete:

Timing - if the spark happens too soon, the chamber pressure may rise too high and detonation results.

Gasoline - if the gasoline burns to quickly (a too-low octane rating), high pressure and detonation are likely.

Glowing objects - a piece of carbon, a too hot spark plug or other glowing object can start burning too soon. Pressure rises too high and detonation can happen. Cranking pressure - Any given combustion chamber has a maximum pressure (before the spark is struck) beyond which detonation is likely.

High engine temperatures - High chamber temperatures raise cranking pressure and promote detonation.

Lean jetting - Weak air/fuel mixtures can result in very uneven mixtures within the chamber, uneven burning, pressure spikes and detonation.

Note that each of these possible causes are relative. That is, there is no absolute timing, mixture strength or ignition timing that is going to guarantee detonation. Equally, there are no absolute settings that guarantee that detonation does not occur.


Spark KnockMotorcycle manufacturers, Harley-Davidson included, spend a great deal of time and money fine tuning their engines to eliminate or nearly eliminate detonation. When we change the engine design in the direction of detonation by, say, raising the compression pressure with domed pistons or milled heads, we increase the chance of detonation actually occurring.

Gasoline quality helps determine whether or not an engine is going to detonate. The higher the octane rating, the lower the chance of detonation.

Modified engines often have had several engine design changes that, combined, increase the likelihood of detonation. High compression pistons, thin head gaskets, some alternative ignitions, some exhaust system designs, etc.

Stock street bike carburetion is very lean for emissions purposes. When the air cleaner and/or exhaust system are replaced by less restrictive components, this stock jetting becomes impossibly lean. The engine does not run well and detonation is likely at some throttle settings. Re-jetting or wholesale carburetor replacement (Mikuni!) is the cure for this particular problem.

If one fits high compression ratio pistons together with an early closing (mild) cam, the cranking pressure may become high enough that serious, engine-deadly detonation is likely. How much is too much you ask?


Spark KnockWell (Rule of Thumb here), Evolution engines are fairly safe against detonation if the cranking pressure remains at 180 psi or less. The TC88 motor can dodge detonation if the pressures remain at 190 psi or less. Keep in mind that these maximums are for fairly stock engines; no porting, no chamber work and no squish areas.

A well shaped combustion chamber with squish effect is much less likely to detonate than most stock examples. The main reason the TC88 engine can withstand higher cranking pressures than the Evo is its better chamber design.

Cranking pressure here refers to the number one gets by conducting a normal compression test. This test is done by removing the spark plugs and fitting a compression gage in one of the spark plug holes. The throttle is then held open and the engine cranked with the starter until the gage needle stops climbing. The resulting number is the cranking pressure.

Ignition systems are important. If the spark plugs fire too soon, the combustion pressure may rise too quickly bringing on detonation. The main reason for having an advance curve built into an ignition system is to avoid detonation. The correct timing for any given engine design (and state of tune) varies with rpm and throttle setting.

Hot spots is more than a night club. If your engine has been running rich or burning oil, it may have thick bits of burned-on carbon. This carbon build-up can literally glow and, under the pressure of compression, start burning before the spark is struck. This leads to severe pressure excursions and, often, detonation.


Spark Knock

Lean carburetion can lead to detonation. Uneven combustion in over-lean air/fuel mixtures can escalate pressures and bring about sudden explosive burning. Also, lean mixtures elevate chamber temperatures which, as you now know, can lead to dreaded detonation.

If all this leads you to think that your engine is in imminent peril, then we have succeeded. Detonation is a terrible thing to happen to your expensive Harley engine. The pressures of those explosive events can be enough to hammer rod bearings, pistons and rings into useless junk.

If you hear the tell-tale ringing of detonation next time you open the throttle on a hot day or at low rpm or after a tank of questionable gasoline, back off the throttle and ride carefully until you can find and render harmless this demon visiting destruction upon your motor.


Poor Mileage6: Poor Mileage

"Normal" fuel mileage normally varies somewhat depending upon a number of factors. An average range for an FXD-series Harley is: 45-51 miles per gallon at 65 mph on a flat road with no wind. The large touring models typically deliver about five miles per gallon less. Fuel mileage of less than 40 mpg at a steady 65 mph (flat road, no wind) indicates a possible mechanical or tuning problem.Common causes:

Choke cable installation:An incorrectly installed choke cable can lead to poor fuel mileage.Carburetor tuning: An incorrectly jetted carburetor can lead to both poor fuel mileage and performance.Speed: Fuel consumption increases dramatically with speed.Head wind: Fuel consumption increases when riding into the wind.Weight: Motorcycles require more fuel when climbing.Size: Larger (touring) models create more wind drag.Engine efficiency: Highly developed engines use fuel more efficiently. Poorly tuned ones do not.


Poor MileageChoke cable installation:

There must be some free play in the choke cable to ensure that the starter (choke) plunger is fully closed. If the choke is held even slightly open, poor mileage, sluggish performance and fouled spark plugs may result.

Harley choke cable

If you are using the Harley choke cable (the word Choke on the knob is white), use this procedure to determine if the choke is closing completely:Pull the choke knob out fully. Loosen the friction nut just enough to allow the choke shaft to move freely. The friction nut is located behind the choke knob. It is thin and has ridges around its outer edge like a coin. If you turn the friction nut out too far, it will interfere with your ability to detect free play in the choke. Now, move the choke knob in fully. Gently pull the knob out. There should be a small amount of free play before you feel the tension of the choke return spring.


Poor MileageIf there is no free play:

Check the routing of the cable. The stock Harley cable is very stiff and tends to bind in the metal elbow at the carburetor-end of the cable. The end of the cable slips into the metal elbow and can jamb. The joint (cable/elbow) is hidden by a rubber cover. Push the cable end fully into the elbow.If this does not cure the problem, it is possible that the choke cable assembly was not assembled correctly. You must use the Mikuni choke plunger and spring with the Harley choke cable. If you install the complete Harley assembly (cable, plunger and spring), the Harley plunger will not seal and the air/fuel mixture will be very rich, especially at idle and low throttle settings. Mikuni choke cable:

The Mikuni choke cable is identified by the small brass bump in the center of the knob. Mikuniís cable is much more flexible that the stock Harley cable and seldom jambs. However, it is possible that its length adjustment can be incorrect in a particular installation. Check for free play by gently pulling the knob. It should move freely for a short distance before the force of the return spring is felt. Even a slight amount of free play is enough. If there is no free play, check the routing of the cable to make sure that it is not kinked or pinched by other components. If necessary, peel the rubber cover back and adjust the length of the cable to introduce a small amount of free play.


Poor MileageCarburetor tuning:

Mikuni HSR42/45/48 carburetors are jetted to meet the requirements of the great majority of engine tuning setups. The HSR-series is very tolerant of engine tuning variations. However, it is certainly possible that minor tuning adjustments may be desirable to achieve maximum performance and/or maximum fuel economy with some engine component combinations. Normal highway cruising speeds (65 mph/ 100 kph) require rather low throttle openings, generally less than º throttle. Air/fuel ratios in this throttle range are controlled by the pilot circuit together with the jet needle and needle jet. Thus, poor fuel economy at normal cruising speeds should be addressed by altering or adjusting these parts.

Pilot system:

The pilot circuit has one replaceable part and one adjustable part. The pilot jet is replaceable and the pilot air screw is adjustable. If the jet is too large or the air screw is in too far, the air/fuel mixture may be too rich. However, it is very unlikely that the pilot jets installed at the Mikuni factory (#20 or #25) can cause a dramatic loss of fuel economy. Some HSR45s are fitted with #35 pilot jets and these may be too rich for well-tuned engines. See the tuning manual elsewhere on this website.


Poor MileageJet needle:

There are four different jet needles. Their part numbers are: J8-8DDY01-95, -96, -97 and ñ98 (J8-8CFY02-xx for the 45 & 48). We commonly refer to them as a ìdash 97, dash 98, etc.î The current standard jet needle is a ì-97.î

The only difference between each needle is the diameter of the straight part of the needle. This is the portion of the needle that controls air/fuel mixture strengths between idle and approximately º throttle. So, when a mixture change needs to be made in this range, it is necessary to exchange jet needles. Raising or lowering the needle has no effect on mixtures below º throttle.

The jet needle is both adjustable and replacable. Its height can be adjusted (via an ìEî clip and five grooves) to change mixture strength between º and æ throttle. Lowering the jet needle leans the mixture and raising it richens the mixture.

Main jet:

The main jet becomes the main fuel control at approximately 3/4 throttle. The main jet has no effect on fuel mileage under any but the most extreme riding conditions.


Poor MileageSpeed:

Fuel consumption increases dramatically with speed. For instance: If you wish to double the speed, your engine must produce approximately eight times as much power. Thus, if 20 horsepower gets you 100 miles per hour, you'll need 160 HP to go 200. From this relationship it is easy to understand why fuel economy drops so dramatically between normal cruising speeds (65 - 75 miles per hour in America) and higher speeds around or above 85 mph.

Head wind:

Fuel consumption increases when riding into the wind. If you ride at 60 mph with a 20 mph headwind the fuel mileage is be better than if you were going 80 mph with no head wind. However, there is still a significant loss of fuel economy.Weight:

Heavy motorcycles require more fuel when climbing. This is simple to understand; the more weight lifted, the more energy (fuel) needed to lift it. Thus, a 20 percent heavier motorcycle requires about 20 percent more fuel to climb a mountain at a given speed than the lighter machine.


Poor MileageAn engine that has been modified to perform best in a higher-than-normal rpm range may suffer a dramatic loss of fuel economy if it is operated under load at an engine speed below its design minimum.

Size:

Larger (touring) models create more wind drag.

Engine efficiency:

Highly developed engines use fuel more efficiently. Poorly tuned ones do not.


Manifolds

7: Which Manifold?

The great majority of HSR installations use the stock intake manifold that has been fitted to all Big Twins and Sportsters for more than 20 years. This manifold is an excellent performer. It is reliable, has excellent airflow and is available. We recommend its use.

However, Should you choose to fit a Mikuni HSR45 or 48, you must fit a manifold other than the stock one. Also, if your engine has larger ports (the large S&S motors for example) or the heads have a non-standard spacing (many S&S strokers or some other clone engine designs), then you must fit a non-standard manifold.

Mikuni produces an alternate manifold design. Our manifold's two piece construction allows us fit different rubber flanges that accept 42, 45 or 48 millimeter HSR carburetors.

All current (May 2002) Mikuni manifolds are machined to fit stock (40mm) intake ports and cannot be used with large (usually 45mm) ports. Also, they are machined to fit engines with stock cylinder head spacing only. We make both Evo and Twin Cam versions of our manifold.

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