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How to Diagnose Carburetor Problems

Rich, Lean or Out of Adjustment?

By

How to Diagnose Carburetor Problems

Keihin carburetors

Photo copyright John H. Glimmerveen

Before attempting to fix a carburetor problem, it is very important to come up with the correct diagnosis.

Carburetors are relatively simple devices. Their primary function is to deliver the correct amount of fuel/air mixture at a given throttle opening (as selected by the rider). However, as with all mechanical devices, carburetors will wear over time and will also require periodic tuning and service.

Carburetor problems generally fall into three areas: rich mixture, lean mixture, and incorrect adjustment. Diagnosing carburetor problems is relatively easy and follows some telltale symptoms.

Three Carburetor Problems

1) Rich Mixture means 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 machine is at idle

  • Uneven running (will often slow from regular idle rpm's and stop)

2) Lean Mixtures means the carburetor is delivering too much air. Typical symptoms of a lean mixture are:

  • Backfires as the throttle is closed (primarily during coast-downs)

  • Lurching acceleration

  • White or light grey spark plugs

  • Requires excessive amounts of choke to run/start

  • White or light grey muffler end pipes

  • Bluing (on chrome systems) of the exhaust header down-pipes

3) Incorrect Adjustment applies to carburetors that have incorrect adjustment of the air/fuel screw and the balance between two or more carburetors - where fitted. Incorrect adjustment can produce any of the previously noted symptoms. On multi-cylinder 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

  • Misfires and/or backfires

Correcting Carburetor Problems

Lean Mixtures: This condition is generally caused by the owner fitting 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.

The carburetor shown in the accompanying photograph has a low speed 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).

Rich Mixtures: This condition is primarily caused by dirty air filters, but it could also result from the owner fitting replacement exhausts and/or carburetor systems. If the fuel level is set too high in the float chamber, a rich mixture will result.

Incorrect Carburetor Adjustment: 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 multi-cylinder balancing screws are the items most prone to self-adjust during normal operation and often require periodic corrections.

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