Hobbies Cars & Motorcycles How to Diagnose Carburetor Problems in Your Motorcycle Rich, Lean, or out of Adjustment? Share PINTEREST Email Print Wiki Commons Cars & Motorcycles Motorcycles Restoration & Repairs Motorcycle History Buying & Selling Cars Used Cars SUVs Trucks ATVs & Off Road Public Transportation By John Glimmerveen John Glimmerveen is a former competitive motorcycle racer. He later worked as a race technician for several international race teams. our editorial process John Glimmerveen Updated August 04, 2018 Carburetor problems generally fall into three areas: too lean a mixture, too rich a mixture, or an incorrect adjustment. Luckily there are telltale symptoms that will tell you which problem you're dealing with. Carburetors are relatively simple devices. Their primary function is to deliver the right amount of fuel/air mixture at a given throttle opening (as selected by the rider). However, as with all mechanical devices, carburetors require periodic tuning and service, and will eventually wear out. Before attempting to fix a carburetor problem on your motorcycle, you need to come up with the correct diagnosis. Too Lean When a carburetor is running lean, the fuel-to-air ratio is off because the carburetor is delivering too much air. Typical symptoms of a lean mixture are: Backfiring as the throttle is closed (primarily during coast-downs)Lurching accelerationWhite or light gray spark plugsRequiring excessive amounts of choke to run/startWhite or light gray muffler end pipesBluing (on chrome systems) of the exhaust header down-pipes Too Rich When a vehicle is running rich, the fuel-to-air ratio is off because the carburetor is delivering too much gasoline. Typical symptoms of a rich mixture are: Poor fuel economySluggish accelerationChoke not needed from cold startsSooty or black spark plugsSooty or black muffler end pipesStrong smell of gasoline when the machine is at idleUneven running (will often slow from regular idle rpms and then stop) Out of Adjustment When the carburetor is out of adjustment, it means the air/fuel screw and the balance between two or more carburetors need to be adjusted. Incorrect adjustment can produce any of the previously noted symptoms. On multicylinder machines, with separate carburetors for each cylinder, the following symptoms are typical of an adjustment problem: Poor overall performanceRattling sounds from the clutchEngine tends to stall easilyErratic accelerationPoor fuel economyMisfiring and/or backfiring Fixing Lean Mixtures This condition is generally caused by the incorrect fitting of after-market accessories such as exhaust systems, air filter systems, or replacement carburetors of a different type or size. In addition, if the fuel level in the float chamber is set too low, insufficient fuel will be drawn through the main jet. Some carburetors have a slow speed fuel adjusting screw that regulates the fuel/air mixture in the lower rpm range. Others have an air adjusting screw. Turning this screw clockwise will reduce the amount of air entering the carburetor, and will, therefore, richen the mixture (refer to a shop manual for correct settings). If no changes have been made to the bike, and it previously ran well, a lean mixture can be traced to a leaking inlet manifold or leaking exhaust (often at the interface of header pipe and cylinder head). Fixing Rich Mixtures This condition is primarily caused by dirty air filters, but it could also result from the incorrect fitting of after-market accessories such as exhaust systems, air filter systems, or replacement carburetors of a different type or size. In addition, if the fuel level is set too high in the float chamber, a rich mixture will result. To remedy, clean your air filter, have your mechanic check the exhaust and carburetor fittings, or both. Fixing Incorrect Adjustments This situation is mostly caused by poor maintenance. With the inherent vibration of all engines, carburetor parts—primarily adjusting screws—tend to rotate, and therefore change their positions. Low-speed running jets and multicylinder balancing screws are the items most prone to self-adjust during normal operation and often require periodic corrections.