QuestionI have a Honda 1980 CB 650 that I have had since new. The bike has developed a pesky problem of leaking thru the overflow valves on at least two carbs. They are Keihin throttle valve carbs. Thinking the floats may be "sticking" I removed and cleaned them but that didn't fix the problem. In fact they seem to leak more now. The gas starts to leak as soon as the fuel valve is in the flow position..regardless if the engine is running. The floats appeared to be in good condition but the float bowl gaskets did seem a little worn. Gas does not leak at the gaskets however, only thru the overflow. The float valve needle also appeared to be in reasonably good condition.
The bike seems to have a tough time idling without choke. Most of the time it will quit as soon as choke is off. It also does not appear to be hitting on the same cylinders as where the overflow leak is coming from. I suspect that may be a timing issue? Or perhaps all of this is related?
I suspect there is a carb problem of some kind but I am not sure where to start in terms of replacing/cleaning parts.
Would greatly appreciate any words of advice.
AnswerBryan, overflow problems can be caused by numerous issues.
1. High float level. If the floats are plastic, you can't adjust them. If you have metal, brass floats, you can adjust the little float needle tang on the middle of the float. 12.5mm is the correct distance.
2. Worn needle/seat. If the needle has a groove in the area where it normally contacts the seat, then they will often leak there. Some needles are rubber tipped and are less apt to wear and leak there, but they can still deteriorate after 25 years.
3. Needle Seat (float valve seat) is not sealing into the carburetor body. Most later carburetors have O-rings that are part of a packing set, that seal the seat to the carb body. They shrink and sometimes crack, causing fuel to drool down the side of the seat, filling the float bowl.
The seats should be removed and checked for debris, backing up behind the seat, which can cause fuel starvation on that cylinder.
4. Rarely, but sometimes, the overflow tube gets cracked from old fuel etching into the brass tubing.
If the carb is flooding, the spark plug for that cylinder is getting fuel fouled, which makes it not run well on the affected cylinders. Fix the fuel leaks, put in a fresh set of plugs and it should be fine again, assuming even compression in all cylinders. Do check the valve clearances, to be sure that you have sufficient clearances insuring good compression readings.
These bikes have electronic ignition, which is generally pass-fail. There is a mechanical spark advancer behind the pulse generator plate that can get sticky, causing timing issues, which result in irregular idle and/or poor performance. Damaged intake manifolds can cause air leaks that will affect engine performance. Taking carbs on and off these bikes is a real pain, so be careful that everything is back where it belongs. I use silicone spray to help lubricate the connections of the carb and manifolds and the air box to carb connectors.
Be sure that the charging system is all fully functional. There are carbon brushes that can wear down and cause the charging system to stop charging altogether.
Go to http://www.powersportspro.com/Partsfish/login/
to see online microfiche illustrations of some of these items.
Bill Silver
