Tuning a carburetor is not the mysterious science many racers believe it to be. One only needs to establish a basic knowledge of the identification and function of carburetor components as well as how they work together to do the job well.
The main jet size should be increased or decreased by one to five sizes and the engine tested until its power is maximum.
Symptoms of Improper Mixture
If your machine exhibits one or several of the symptoms listed below, it may need carburetor tuning. Before attempting any changes, however, make sure that everything else is in good shape and properly tuned. spark plug, make sure the ignition timing is correct, service the air cleaner element and decarbonize the muffler.
If your machine has run properly on a certain track in the past and then starts running poorly with the same carburetor settings, the problem is almost certain to be elsewhere; changing the carburetor settings in such a case would probably be a waste of time.
With pinging or rattling, make sure that the gasoline is fresh and the octane rating sufficient. You might also try a different brand of high-octane gasoline.
Making Adjustments
If you note a particular symptom of too rich or lean mixture in a specific operating range, use the chart to determine which components need adjusting. Use the following information to decide what changes to make.
Main Jet
The main jet has its greatest effect in the 1/2-to-full-throttle range. The number stamped on the bottom or side of the main jet indicates the size of the hole in the jet which meters fuel. The larger the main jet number, the bigger the hole and the more fuel will flow; hence, larger numbers mean richer jetting; smaller numbers mean leaner jetting. Make main jet changes one size at a time.
WARNING
Gasoline is extremely flammable and can be explosive under certain conditions. Never smoke while working on the carburetor. Make sure the area is well ventilated and free from any source of flame or sparks; this includes any appliance with a pilot light.
Jet Needle
The jet needle and needle jet together have their greatest effect in the 1/4-to-3/4-throttle range. The needle moves in and out of the needle jet. Since the needle is tapered, its position in the needle jet deter- mines the amount of fuel allowed to pass through.
Clip Position
Slow Jet and Air Screw
Test Runs
With all carburetor settings on standard, warm up the engine, run two or three laps of the course and examine the spark plug condition with varying throttle openings.
Fuel/Air Mixture Adjustment
NOTE
lf the fuel/air mixture is too lean, the engine tends to overheat and may seize up. On the other hand, if the fuel/air mixture is too rich, the spark plug gets wet, which causes misfiring. The richness of the mixture varies depending on atmospheric conditions (pressure, humidity, and temperature).
NOTE
For the following recommendations to be applicable, you must use the standard settings as a base. Do not change any of the settings until you have determined which are necessary. All specifications are based on the use of the specified fuel and oil.
Main Jet Size Selection for a Correction factor of 0.95
Change the jet needle clip position and air screw opening according to the appropriate chart on the previous page.
EXAMPLE: For a correction factor of 0.95, leave the needle clip position and turn out the air screw an extra half turn
Jet Needle Clip and Air Screw Setting for a Correction Factor of 0.9
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