The carburettor in a scooter provides the critical mixture of fuel and air that feeds into the engine. In turn, the engine uses this mixture with a spark plug to create combustion and the energy that runs the engine parts. Understanding how to adjust and tweak a carburettor and its parts can help solve problems with engine sputtering, flooding (too much gas), running lean (too little gas), and can improve performance. Much of the process involves simply understanding how to adjust various parts.
Locate the idle screw on the side or top of the carburettor. Find a twist dial type of screw or one that moves via screwdriver. With the engine on, twist the screw 1/4 turn. Notice the change in engine speed pick up. Reduce the engine idle speed by twisting the screw in the opposite direction. Find the speed where the engine ticks over consistently without sputtering or racing fast. Keep the needle at this position for optimum idle.
Open the carburettor up to adjust mid-range and high-speed performance. Remove the carburettor from its mounting on the scooter. Unscrew the carburettor body parts with a screwdriver or crescent wrench to access the internal jets and needles. Examine the jets and needles and refer to your owner's manual or shop manual for proper jetting in relation to the engine size. Increase the jets' size with bigger jets to provide more fuel or reduce the fuel flow with smaller jet sizes. Close up the carburettor with new gaskets and reinstall it on the scooter.
Locate the air/fuel mixture needle on the side of the carburettor. Adjust this needle by twisting it with a screwdriver. Set it to the position specified in your owner's manual or shop manual in relation to the engine size.
Examine the choke and throttle cables attached to the carburettor. Identify if there is any slack in the throttle cable. Remove the slack by tightening the throttle cable installation in the handle and the carburettor. Check to make sure the tighter cable response is better and faster.
Test-drive the scooter after making the adjustments. Drive it around slowly at mid-range speed then at fast-range speed. For each speed test, drive for about five minutes then pull over. Use a socket wrench and pull out the spark plug. Confirm the plug looks chocolate brown on the tip. Change the carburettor adjustments to slightly less fuel if the tip is oily black. Change the carburettor settings to more fuel if the tip is chalky white. Continue adjusting and testing until the spark plug tip looks chocolate brown in colour.
Having a number of spare carburettor jets and gaskets for your scooter allows you to make adjustments on the road should you be on a long trip with altitude changes.
Making carburettor adjustments can affect the performance of your scooter, sometimes negatively compared to factory settings. If you are not sure how to go about making changes, either let a dealer make the changes for you or work with an experienced scooter mechanic to show you the steps.