'80 GS750 VACUUM PETCOCK ISSUES - CURE (or at least a diagnostic tool)

[DV8]

I just went through this and thought I'd share with everyone......

My GS750 w/cv carbs came out of hibernation this spring with dirty carbs for some unknown reason (never did before )

So while putting off cleaning them out and hoping they would fix themselves by running a couple-few tanks of fuel through, it developed a new problem - automatic petcock shutting off randomly. I could stop and pull the hose, apply an organically fueled vacuum sorce (me) to the petcock and fill the carb bowls, and off I'd go again. Maybe for 2 miles, maybe for 50, then same thing.

This gets old. Fast. So while at work, I begin brainstorming and realize that the same carb that provides the vacuum signal is the same one that is dirty everytime I drain the bowl (which has been frequently, see sentence one..). So I figure that hey, maybe its crudded up so much that it can't provide an adequate signal to keep the fuel flowing.

So let's hook up to another carb for the signal, right? Wait, there is only a fitting on the second carb, none of the others are machined for a vacuum port.

So I pull the carb synchronizing hex bolt from #1 on the head, find a grease zerk in the parts bin with the same threads, gut it, and install. Grab a new piece of fuel hose, run from the zerk to the petcock vacuum port, and plug the original hose with a bolt. Looks redneck but this is only a test, dig?

And guess what? My theory seems valid - no stalling/running out of fuel randomly now. Yippee!

So now I can go back to putting off pulling the carbs and doing a proper cleaning..........

Hope this helps someone, Cheers!

[Dave8338]

Are you sure that it's not just crud getting through the fuel screen in the tank? A screen at all? It wouldn't be the first thread where the fuel screen is either missing completely or is laying in the bottom of the tank.

Just another thought...

[80GS750]

wow man thats some mcguyver thinking there..... but hey it works

i like it

[Killer2600]

Nice fix but if your #2 carb is running sh*tty enough to not give adequate vacuum for the petcock then your #2 cylinder has to be running like crap. I'd fix the actual problem rather than bubble gum and duct tape the thing when it's really not needed (the bike isn't your only vehicle I would hope)

[robertbarr]

If your engine is running -- even if #2 isn't firing -- there is flow through that carb body. If there's flow through a venturi, there will be vacuum.

The only problem that could prevent the development of vacuum would be a severe valve problem -- and I can't think of one that could prevent airflow into the carb...

It has to be just a coincidence. (And your bike's petcock doesn't have a Prime position??)

Good MacGuyver work, though.

[chef1366]

Petcock vacuum and a dirty carb have nothing to do with one another. The vacuum is drawn directly from the cylinder itself and not the carb. You either have a valve adjustment issue which causes bad compression or bad rings which cause bad compression or more than likely a bad petcock.