Harley Davidson Carburetor

Motorcycle carburetors look to be a very complex animal at first glance.  With a little reading and trial & error, you can tune your bike for maximum performance.  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 on a two stroke engine goes up (or goes down on a four stroke engine), a low pressure is formed inside the crankcase (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.

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).

Carburetor troubleshooting is simple.  With basic principles and concepts understood you are half way to getting the optimal results from your machine.  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.

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.

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.  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.

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).  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.

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