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.
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 acceleration
- White or light gray spark plugs
- Requiring excessive amounts of choke to run/start
- White or light gray muffler end pipes
- Bluing (on chrome systems) of the exhaust header down-pipes
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 economy
- Sluggish acceleration
- Choke not needed from cold starts
- Sooty or black spark plugs
- Sooty or black muffler end pipes
- Strong smell of gasoline when the machine is at idle
- Uneven 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 performance
- Rattling sounds from the clutch
- Engine tends to stall easily
- Erratic acceleration
- Poor fuel economy
- Misfiring 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.