Carb bowl overflow

[vdavidoff]

Some of the carb bowls on my 74 GT550 are overflowing if the bike sits for a day or two. Fuel is coming out of the overflow ports at the bottom of the bowls. Mainly the middle carb, but sometimes also the left carb. I want to make sure I understand what could be causing this.

If I understand these carbs correctly (and I might not), the only things that could be causing this are:

* Floats that are sinking/damaged or set to the incorrect height
* Leaking valve seats/damaged needle valves

Is that really all, or am I missing other potential causes? I have a suspicion that the diaphragm in my petcock isn't sealing well, but even if that's the case, any overflow that might occur because of that would come from somewhere other than the overflow ports on the bottom of the bowls, right?

I took the carbs off and I thought I adjusted the floats properly - measured from the bowl gasket to the top of the float with the carb upside down, but just touching the needle valve - but the problem persists.

Thanks!
Andy

[GreenMachine]

Might be wrong, but doesn't fuel expand a touch as it heats up, slightly pressurising the float bowl and causing a drip? Probably wouldn't cause a massive drip but might force a few drops out.

[vdavidoff]

Thanks for the idea. Just to clarify: the overflow happens as the bike sits, not once it is started. Interesting that you mention temperature - this does seem to be worse when the temperature swings colder (we've had quite a few temperature swings in the past week or two). Maybe that's just coincidence though, or related but not in the way you described.

I'll also add that I am assuming I should, generally speaking, never have overflow like this. If this is considered normal, then I guess I don't have a problem.

Thanks again.
Andy

[Cliff]

If the needles and seats don't seal 100% on the floats AND the petcock isn't closing or sealing 100% to shut off the fuel, then the weight of the fuel in the tank will cause your leak. I'd repair/replace the petcock first.

[vdavidoff]

Cool, thanks Cliff.

Andy

[Suzukidave]

I would check the petcock first , if it is one of the stock vacuum type and the fuel cutoff O ring has failed it will allow gas to flow all the time to the carbs . A easy test is to pull off a fuel line from the petcock and if it drips there's your problem .

[vdavidoff]

I took apart the petcock and to my surprise it was full of nasty, nasty stuff, including where the diaphragm plunger seals off the fuel. I say I was surprised because I thought I got that all taken care of when I originally de-rusted the tank and cleaned the petcock. I guess there was more crud in the tank I missed.

Anyway, for the past 2 days it has been sitting without dripping fuel. I was also getting some dripping with the petcock set to main that went away (at least shortly) after cleaning it all up. I guess if it stays dry for the next few days, I'll feel like it's fixed.

Thanks again, all.
Andy

[Cliff]

Good to hear we helped!

[vdavidoff]

(Came across this post again while doing a related search)

Figured I'd add that this problem showed up again. I rebuild the petcock completely including the rubber o-ring on the diaphragm, which I think was the problem all along. I didn't even realize the o-ring that was there was actually an o-ring. It was deformed and hard so that it looked like a piece of molded plastic. Things are definitely no longer leaking with it replaced. Though now it seems that the vacuum from the carbs isn't enough (at idle) to always keep fuel flowing with the petcock in On or Res.

Andy

[Suzukidave]

Did you check for vacuum leaks .. those old tubes split pretty easy ( unless you already put new ones on ) also its so easy to hole the diaphram taking apart the petcock .

[vdavidoff]

Thanks for the advice, but that didn't seem to help. The rubber looked okay but I went ahead and replaced it anyway. No change. I guess maybe something is up with the diaphragm and now that it's actually sealing, it's not pulling open far enough. Or something. In any event, this project has run its course. The bike still has a few issues but it basically runs and rides, so I'm going to sell it.

Andy