How to Adjust the Air-Fuel Mixture Ratio of a Underbone Motorcycle Carburetor?
2 Answers
Remove the wind baffle obstacles outside the underbone motorcycle. Loosen the oil drain screw at the bottom of the carburetor to drain all the dirty fuel inside, and replace it with fresh gasoline. Rotate the idle speed adjustment screw of the carburetor to keep the vehicle's engine running smoothly. Rotate the air-fuel mixture adjustment screw of the carburetor until the engine reaches its highest RPM. At this point, rotate the idle speed screw again to reduce the engine speed to 1000 RPM for stable operation, completing the air-fuel mixture adjustment.
When working on underbone motorcycles, I often encounter issues with the carburetor's air-fuel mixture being off. The main adjustment is done using the mixture screw. First, ensure the bike is warmed up—let it run for a few minutes to heat the engine. Then locate the small screw on the carburetor (turning counterclockwise leans the mixture, clockwise enriches it). Use a screwdriver to turn it slowly, pausing every quarter turn to listen to the engine—rough idling or stalling means it's too lean, while black smoke or sluggish acceleration suggests it's too rich. It's best to also check the spark plug color: a tan-brown hue is ideal, too black indicates a rich mixture, and too white means it's too lean and lacks power. Don’t forget to clean the air filter—a clogged one can make the mixture too rich. DIY adjustments are simple, but be careful not to turn too quickly, as it can disrupt the idle. If unsure, it’s wise to ask someone experienced for help to avoid damaging engine parts or wasting fuel.
