At low throttle openings, the carburettor uses the pilot (‘idle’) jet circuit and the carburettor needle to set the fuel/air mixture. If you ride behind a scooter that smokes heavily at traffic lights, it is likely that it is simply set too rich. Too much fuel and oil is being added to the air at low throttle openings resulting in incomplete burning and tons of smoke.

You can clearly hear when an engine is too rich on low throttle. Instead of a clear ‘zing’ as you open the throttle, the revs respond slowly with an audible slow, chuggy ‘bleurggh”.

Fixing this problem can be as simple as setting the air or mixture screw on your carburettor correctly (the one that isn’t the tick-over screw). Often this can be done by ear. Adjust so that the engine has more ‘zing’ just as you start to open the throttle, but not so far that the engine takes longer to settle back to tick-over when you shut the throttle.

IMPORTANT: If the air or mixture screw is at the limit of its adjustment range (check the manual for your type of carb) you may need to change the size of your pilot (‘idle’) jet.

The other issue that causes progressively smokier running on 2-strokes is carburettors that are simply worn-out through years of being vibrated to death and inhaling road grit. A simple check is to look at the condition of the throttle slide. If it shows clear scratch marks or is wobbly in the carb body then a new slide, carb needle or maybe even a complete new carb may be required.