Scooter and small engine carburettors come in many shapes and sizes. For the purposes of performing adjustments however, there are three basic types. There are those with a fuel adjustment screw, those with an air adjustment screw and those that have both adjustments. On smaller carburettors, the latter type are rare. Adjustment screws are often difficult to access, and in the United States street legal motorcycle screws are capped or plugged to prevent amateur adjustments that can affect emissions. These caps are uncommon on smaller engines except in California, where emissions laws are stricter. These caps are easily removed for repair.
Find the adjustment screw. Either consult your manual, or look for a small screw in a recessed hole with a flat head top. Occasionally, the screws are located on the bottom of the carburettor. Using the long Phillips screwdriver, loosen the clamps on the intake and air box boots, and then rotate the carburettor so it is easier to see. On street legal scooters, look for a copper coloured cap or plug on either the engine side or the air box side of the carburettor. If necessary, remove the plug.
Identify if the screw is an air or fuel screw. The air screw will be located closest to the air box, away from the engine side of the carburettor. The fuel screw will be located on the engine, or intake side of the carburettor. Turning the screw clockwise reduces the amount of air or fuel that will reach the engine. Turning the screw counterclockwise will result in more fuel of air entering the engine.
Tighten the intake and air box boot clamps if you loosened them before. Check the air filter before you begin. Never adjust a carburettor to a dirty air filter. If you have an in line fuel filter, change it before you begin as well. Start the engine, and let it idle for a moment allowing the idle to stabilise. Be sure that the idle is adjusted to the proper range according to manufacturer specifications.
Begin adjustments, making small changes. Never adjust a screw more than half a turn at a time. Use the stubby flat screwdriver to reach screws on the bottom of the carburettor. Carburettor adjustment tools are available. These are right angle screwdrivers that allow for easy adjustments of hidden screws. Blip the throttle between adjustments, and listen to the engine. A low gurgle means the engine is running rich, or has too much fuel. An engine that hangs up, and does not return to idle immediately is too lean. It is better to be a little rich than a little lean. A severely lean engine can seize easily, and running an engine even a little lean long term can result in piston damage.
Test ride the motorcycle after you are satisfied with how it sounds at idle. If you cannot adjust the fuel and air mix to your satisfaction, re-jetting may be needed. After re-jetting, always perform carburettor adjustments again.
The best way to perform carburettor adjustments precisely is with an EGA machine, which measures carbon dioxide emissions and raw fuel emissions. These machines are very expensive, but more and more shops have them. An hour with an EGA machine can give you a perfect carburettor tune for your altitude and climate area. Check with local shops to see if they offer this type of tuning.
Fuel vapours are flammable and may be hazardous to your health. Always work in a well ventilated area away from heat and flame.