Carburetor adjustment does not require an engineering degree, but some mechanical ability is necessary. Whether it is a stock or aftermarket carburetor, you will need to consider the temperature, altitude and humidity as these can have an effect on how well it is set up. These environmental factors determine how the fuel-to-air mixture runs through the carburetor. For example, in a higher altitude there is less air for your motor and the fuel-to-air mixture will be richer. Thus, your carburetor will need to be adjusted to accomodate less air in the atmosphere.
Adjust the idle speed by locating the small spring-loaded screw which is usually located next to the throttle control. If you have a tachometer, turn the screw until you reach an idle speed between 1500 and 2000 rpm. If you do not have a tachometer, set it for a low idle speed; not so slow that the engine stumbles or stalls, but not so fast that the clutch won't try to engage and move the vehicle forward.
Adjust the fuel-to-air ratio (FTA) by locating the slotted idle mixture screw near the air-cleaner side of the carburetor. With the engine off, turn the screw all the way in counting the number of turns it takes. This is your baseline, if you are not happy with the adjustments.
Turn the screw back to the baseline, start the engine and let it warm up for five to ten minutes. With the engine idling, turn the idle mixture screw clockwise in ¼ turn increments until the engine drops in speed. Count how many turns this takes.
Turn the screw counter clockwise in ¼ turn increments until the engine revs back up and drops again. Count how many turns this takes. Divide this number in half to find out where the optimal point is. Turn the screw in that number of turns.
Fine tune with ¼ turn increments to find the optimal point that satisfies you. You may have to go back and adjust the idle speed screw if the idle is now too fast.
- Jupiterimages/Goodshoot/Getty Images