Carb Overflow -- Vacuum line issue?

[DustoffDave]

A little background first. I bought this bike (1978 GS750C) about a year ago for $200 and it hadn't been registered or even run since 1991. The tank was wrong and there was no petcock. I've since rebuilt everything on the bike gotten the correct tank and petcock and today the goal was to get it started. Well, amazingly, I did, but I noticed that the #4 carb overflow line was dumping fuel and there was a bit of fuel coming back through the airbox as well. I think I may have a vacuum line hooked up wrong to the petcock as the carb dumped fuel after the bike was shut off (and thusly, had no vacuum), but I am unsure as it wasn't hooked up correctly when I bought it and the technical manual I have for it doesn't talk about the lines running from the carbs to the petcock. Also, I made sure that the petcock was in the "On" position and not the "Pri" position. Any advice on this would be much appreciated. Thanks in advance.

[Agemax]

sounds like the float valve is stuck in the carb. need stripping and cleaning or maybe replacing

[DustoffDave]

Hmm, I took all of the carbs completely apart, dipped them, cleaned them, and rebuilt them. Could it just be really out of adjustment?

[Agemax]

if its just one carb i suggest you strip it down again and check all is correct

[DustoffDave]

Works for me. That will have to be next weekend's project...

[oldgsfan]

Is it a new petcock you installed? The problem you're having with the leaking fuel was one I had that turned out to be a bad petcock. Like you, I had already cleaned my carbs when the leak developed.

If it's a new petcock, nevermind.

[sedelen]

Disconnect your fuel line at the petcock and see if you still get fuel flowing from the tank with the petcock in the On or Res position, if you do you got a leaky petcock. Might want to take your tank off to do this or have another fuel line ready to connect and have that routed to something to collect fuel should it leak. With ample fuel in the tank the petcock should shut off completely in Res or On and run freely in Pri.

[DustoffDave]

The petcock is an old one that has been rebuilt, so that is a valid suggestion. That'll probably be easier to check first rather than taking the bowls off.

[koolaid_kid]

I agree with sedelen with one exception: it sounds like you have 2 issues.
The first one is the petcock. As you aware, it should not flow if it is in the ON or RES position.
The second is either a stuck needle and seat or incorrect float level settings. Even if the petcock is in PRI position, the fuel bowls should not overflow. Period. If it does overflow when in PRI postion, you absolutely must remove the fuel bowl and fix the issue.
Think of it this way: PRI is a legitimate position for the petcock. It is to be used when the bowls are empty, for example. So if the bowl overflows, you stand the chance of flooding the cylinder and possibly igniting the fuel outside of the cylinder, which can be detrimental in several ways.

[d19407]

I would like to throw in my $’s worth.

I just fixed the same problem on my 1979 GS850. My experience was as follows:

I rebuilt the petcock, but in my case it did not fix my problem (but that’s okay, I recommend doing it anyway ).

Having rebuilt the carbs just a few months ago (full rebuild and clean – all new jets, springs, gaskets etc) it was painful to do, but I took the carbs off and removed the bowls.

I checked the heights of the floats and found them all to be within limits except that one was slightly different from the others. I also noticed that the same float was not true – that is to say: one of the floats was slightly bent outward and not pointing exactly forward. I checked this float in the bowl and I could imagine a scenario where it may occasionally stick in the bow wall l due to its misalignment.

I adjusted accordingly and that fixed my problem.

Also, while you have the bowls off may I recommend placing the floats in a bucket of water for 20 mins to see if they float. A tiny puncture in a float may also be the cause.

Good Luck.