Gas tank vacuum

[beerbaron]

Hi, I have an FZ600 1988. I've got a intermittent problem since last year. My bike loses mosts of it's power once in a while, it only seems to happen when the bike is on the road and I can't reproduce it, heat has no effect.

So far, I've checked the carbs and they are perfect.
Changed the fuel line
Changed the vacuum line (from the engine intake to the gas tank)
Changed the spark plugs cap
Changed the spark plugs
Verified the coils impedances

I have a few hypothesis about this problem and I would like to discuss it with you guys.

1st- The problem is due to a valve that gets stuck sometimes
2nd- The problem is due to a bad vacuum in the gas tanks making my carbs run dry
3rd- A somewhat related CDI problem

So what do you guys think about it?

BeerBaron

[safety-boy]

This is car logic, and possibly doesn't apply:

I have heard that a clogged rebreather in a car will create a vacuum, contracting the (plastic) tank. When the tank is able to expand, it will suck gas out of the fuel line.

Obviously a lot of that has no bearing on motorcycles, but air would need to get into the tank to replace the lost fuel volume.

Old bikes are fun, aren't they? My '86 Kawa has its occasional quirks - usually just after I tell people how indesructable it is

Good luck with that. I'll have to keep my eye out for more enlightened responses in case I run into the same!

--Dave

[Skier]

Hi, I have an FZ600 1988. I've got a intermittent problem since last year. My bike loses mosts of it's power once in a while, it only seems to happen when the bike is on the road and I can't reproduce it, heat has no effect.

So far, I've checked the carbs and they are perfect.
Changed the fuel line
Changed the vacuum line (from the engine intake to the gas tank)
Changed the spark plugs cap
Changed the spark plugs
Verified the coils impedances

Gas cap vent. Try riding around until the problem happens, then open the gas cap. If you hear a rushing of air into the tank, it's your problem. If it goes away after opening the cap, it's your problem.

The vents can get clogged pretty quickly. I'm actually fighting this problem with my Radian myself, the danged cap will clog itself up in a month or two! I'm currently trying new methods of cleaning that don't involve disassembling the entire bloody gas cap.

[beerbaron]

Thanks guys

[niterider]

If you suspect that a carb float is sticking, carry a tool where you can get to it while riding. When the problem shows itself tap the carbs one at a time to see if the floats unsticks and the fuel resumes the flow. Read this here some time back. Hope this will help find the problem.