Petcock rebuild?

[Kyle M.]

78 GS750.....When I run the bike I have to leave it on prime to keep it running, but then my oil gets gas in it. To get it to work with it turned to on, do I have to rebuild it? What else can I look for?

Also, is there only one vacuum port, the one in the center of the carbs, that I have to hook the petcock to to operate it? It was plugged off when I got it, so I am not sure how it is supposed to go.

I want to get this thing on the road.....I have been resorting it for a few years now. Thank you!!

[bwringer]

Buy a new petcock and ride happy for the next 20 years.

Half the time or more, the rebuild kits are junk, or the carcass of your old petcock is too corroded inside. Since a rebuild kit costs about half of what a new petcock costs, it's not a good gamble.

The fuel line will hook up to the larger nipple on the petcock, and the vacuum line will hook up to the smaller.

Gasoline is flammable, and the petcock is mere inches from your crotch. Do the right thing...

[Kyle M.]

Well, I went out and unplugged the vacuum line from the carb, turned the petcock to on, and sucked on it a little and gas started flowing. I think I might be alright for now, but thanks

[dr_fosg8]

I am not positive but if you run on prime and get gas in the oil you may have a float sticking.

I just replaced my fuel valve due to the vacuum line that opens it drawing gas into the number 2 carb, super rich running then.

Best of luck, Parker

[SqDancerLynn1]

On the VM carbs you have overflow lines on the bottom of the float bowl/ rout to rear & down... Fuel feed connection is between carbs 2&3
Vacuum petcock goes to the fitting on side of #3 carb
Carbs 2&4 the fitting is vented to the rear , up & over the airbox

[Kyle M.]

I am not positive but if you run on prime and get gas in the oil you may have a float sticking.

I just replaced my fuel valve due to the vacuum line that opens it drawing gas into the number 2 carb, super rich running then.

Best of luck, Parker

I just rebuilt my carbs, maybe I need to check my float levels again. If I had a float not working, wouldn't gas come out the over flow?

I read somewhere that if you leave your petcock on prime all the time you will get fuel in you oil, and I assumed that to be true considering how the petcock operates with vacuum like a regulator.

[Kyle M.]

On the VM carbs you have overflow lines on the bottom of the float bowl/ rout to rear & down... Fuel feed connection is between carbs 2&3
Vacuum petcock goes to the fitting on side of #3 carb
Carbs 2&4 the fitting is vented to the rear , up & over the airbox

Thank you, that is how I have it

[SqDancerLynn1]

It should run fine on prime, if you do not turn it back to the on position it can leak into the engine thru the cylinders into the oil

[Kyle M.]

It should run fine on prime, if you do not turn it back to the on position it can leak into the engine thru the cylinders into the oil

Okay, but why and how? You shouldn't leave it on prime should you?

[Zook]

No, you should turn it to the ON position. Engine vacuum will operate the diaphragm which acts like a fuel pump. No vacuum, no fuel. PRIME bypasses this and fuel flows freely.

When left in the PRIME position the fuel flows to the carbs and if the float needles and seats leak the fuel will fill the carb bodies and then into the cylinders through an open intake valve. Then it can do all sorts of things, like hydrolock a cylinder. You hit the starter button and the piston moves upward and hits all that gas-WHAM! Hopefully a rod doesn't get bent.
Or, if the bike sits long enough the fuel leaks past the rings and into the crankcase. By then it has gummed up the rings.

I've had both the hydrolock and I was able to by my GS because the former owner thought it had a holed piston because gas/oil was coming from the carbs when it ran. It was full of gas!