Overflowing Carbs

September 20, 2009 12:13 PM

Help me diagnose a motorcycle carburetor problem.

Recently, my motorcycle suddenly lost power. It still runs, but apparently only on one cylinder.

On the bad cylinder side, gas drips out of the exhaust pipe (where the pipe from the engine meets the chrome). The rate increases with the throttle. Evidently, liquid gas is being pumped into the engine.

I checked the fuel level on the carburetors using a clear plastic tube, and sure enough found that the bad side was way too high, while the good side was just right. "Aha!" I said, "The float or needle valve is bad."

However, the floats are fine (they float, no bent parts, etc...) and replacing the valves did not solve the problem. In fact there wasn't any visible wear, varnish, or anything else that seemed to be wrong with the old valves or any other part of the carburetor for that matter. (I didn't disassemble past the fuel reservoir). The carbs looked squeaky clean, in fact. The fuel filter is clean as a whistle, too.

Yet fuel still drips from the exhaust. Performance is still poor. The fuel level in the carbs is still significantly above the top of the reservoir.

What could it be? How are the carbs filling past the point where the float should shut the valve?

posted by dsword to travel & transportation (9 comments total)

How's the spark plug in the bad cylinder?
posted by knave at 12:45 PM on September 20

Fine. It looks a little fouled due to a rich mixture, but I've even swapped plugs from the two cylinders.

But a bad spark shouldn't cause the fuel level in the carbs to be high in any case.
posted by dsword at 12:49 PM on September 20

Have you confirmed that the liquid is gasoline? If enough were going into the cylinder to pass on through it, you should have a very wet plug, not a fouled one. It's hard to imagine the plug could even fire in that circumstance.
posted by TruncatedTiller at 1:16 PM on September 20

My hubby says it sounds like a problem with your ignition coil. You're not getting a spark to the relevant plug. Check your compression in the bad cylinder. Also, get an ohm meter and check resistance on your coil. (consult your owner's manual for compression specs and resistance specs)
posted by amyms at 1:18 PM on September 20

Check to see if the float drop is in specs.
posted by torquemaniac at 1:39 PM on September 20

Maybe a damaged needle valve and/or seat. Depending on the model of bike, many later carbs don't have a replacable seat. The needle valve is located and is operated by the float arm. Sometimes you can see a wear ring worn into the rubber tip of the valve.
If your bike has a vacuum operated fuel tap, that could be faulty as well.
What make/model is your bike?
posted by Duke999R at 2:32 PM on September 20

Well I see you replaced the valves, so disregard that ;)
posted by Duke999R at 2:47 PM on September 20

Have you checked to see if your bowl vent is plugged? Many carbs have a bowl vent fitting to equalize air pressure within the bowl to outside air pressure, and to vent fumes from the bowl to reduce vapor pressure leading to vapor lock in hot weather. If your bowl vent is plugged, and your engine is still delivering full vacuum, some carbs will pull gas past the float valve, leading to high fuel level and bowl overflow.
posted by paulsc at 4:07 PM on September 20

Is the spark plug getting spark?
posted by gjc at 9:05 PM on September 20