Motorcycle Carburetor Problems
(text and pictures by Mark Trotta)
Only in theory are carburetors simple devices.
Carburetors are designed to mix a small amount of gasoline with the right amount of air so that an engine runs properly. How complicated can mixing two things be? Well, factor in cold starting, idling, and wide-open-throttle, then delivering the correct mixture all the time gets a bit more complicated.
Fuel And Air
To get your classic bike running just right, the carb needs the proper mix of fuel and air. Before you start adjusting your carburetor, remember that no matter how well-tuned it is, it will not make up for bad ignition, wrong spark plugs or incorrectly adjusted engine timing. Make sure all of these are in good shape before changing carburetor jets and settings.
As with all mechanical devices, carburetors wear with usage. They sometimes require periodic cleaning and adjustment. The two most common problems are running too lean or too rich.
- A lean mixture gives an erratic idle, with tendency to stall when not fully warm.
- Too rich a mixture gives an even, rhythmic misfire and a bit black smoke in the exhaust.
- A correct idle mixture gives an even idle and colorless exhaust.
A lean condition exists when the carburetor is delivering too much air. If there is not enough fuel mixed with the air, an engine will run lean, which will cause it not to run it's best, or potentially damage the engine. Typical symptoms of a lean mixture include some or all of the following:
- Lurching acceleration
- Misfires and/or backfires
- Engine stalls easily
- Backfires when throttle is closed (primarily during coast-downs)
- Blueing on chrome exhaust pipes
- White or light grey spark plugs
- White or light grey on end of exhaust pipes
- Poor fuel economy
Fixing A Lean Mixture
Most carburetors have an adjusting screw that regulates the fuel/air mixture in the lower rpm range. Turning this screw clockwise will reduce the amount of air entering the carburetor, and will richen the mixture (refer to your shop manual for correct settings).
Sometimes adding after-market accessories such as exhaust systems, air filter systems or replacement carburetors will contribute to a lean mixture. If no changes have been made to the bike, and it previously ran well, a lean mixture can sometimes be a leaking intake or exhaust gasket.
If there is too much fuel mixed with the air, the engine will have a rich condition. This may cause flooding, bogging, and stalling. It also wastes fuel.
Fixing A Rich Mixture
If the fuel level is set too high in the float chamber, a rich mixture will result. A rich condition may be caused by a dirty or clogged air filter(s). Also keep in mind that both altitude and humidity effect your engine's performance.
Carburetor adjustments should be done with engine at normal operating temperature.
A vacuum leak will allow an engine to suck air in through a small crevice where it shouldn't, which throws off the gas/air mixture. Check and tighten manifold bolts and clamps, as well as carb mounting bolts.
Octane Requirements (rule of thumb)
- 80 octane for 8:1 or less compression motors
- 90 octane for 9:1 compression motors
- 100 octane for 10:1 compression motors
The Effects Of Ethanol Gas
Ethanol is used in today's gasoline as an additive, and E10 gasoline today contains roughly 10% ethanol. Ethanol has a shorter shelf and tank life than gasoline, and can begin to break down in as little as three weeks. Another problem with ethanol is that it attracts water and "breaks down" faster than gasoline.
Ethanol is the same type of alcohol that is found in an alcoholic drink, which works somewhat like a cleaning agent - not good for an older engine. It also increases vapor pressure in gas, which may cause vapor lock in the carburetor. These issues are not so bad with daily transportation vehicles, but with small engines not driven regularly they are problematic.
Fuel System Maintenance
If you store your bike over the winter, untreated fuel may go bad in as little as three months. Longer than that, a fuel system cleaning is recommended.
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