Petcock Test

[gorrbert]

My 83 GS1100GL is not starting, and I wanted to rule out the petcock as a problem. I disconnected the fuel line from the carbs, and while cranking the engine, no gas came out of the fuel line. My guess is that the negative pressure that induces the petcock to open was supplied by the vacum hose, but the petcock, in the run position, failed to open. When in the prime position, fuel flows through the fuel line. In the run or reserve position no fuel when the engine cranks. Should I replace the petcock?
Thanks for any help.

[chuckycheese]

Yeah

Yeah, if the vacuum line is still hooked up and you disconnected the gas line from the gas tank, gas should flow when you turn over the engine. I think you've got it figured out just right.

[tlepac]

Another test is to hook up a line to the vacuum line at the petcock and put gentle suction to it. If gas flows from the gas line then the petcock is working--If not then get a new one. Or just skip this part and get a new one!! Probably easier in the long run. But that is how I checked mine. Not vacuum line from the engine dependent. Just don't over do it with the suction or you will be for sure buying a new petcock!

[Griffin]

Yeah, the above are good suggestions, and check your vacuum line is clear, in good shape, and connected tightly at both ends.

-JC

...and not kinked when it's attached to both the petcock and carbs.

[duaneage]

The engine does not generate a lot of vacuum when cranking, it may not have enough pull to open the diaphram.

Take a clean new hose and hook it to the petcock and use a vacuum pump to pull a vacuum on it. You could also try sucking on it but this is a family website and I'm trying not to get us in trouble.

[Steve]

If it has been a while since the bike ran, the petcock could simply be stuck in the closed position. Using the PRIme setting bypasses the port that is closed by the diaphragm, so I would guess that the plunger is stuck in the port.

I had this same problem a found that the o-ring that the plunger is supposed to seat against to block fuel flow was actually ON the plunger, preventing it from moving. Once I put the o-ring in its proper place, the piston could move freely and the petcock worked properly.

.

[duaneage]

TheBigRed and I opened a petcock up that would not flow on PRI and found there is a little tab which manually moves the diaphragm to open the fuel flow. We were surprised by this since we both thought another path was created but it simply moved the diaphragm just like vacuum would. Some petcocks may be different but it looked just like a standard GS petcock on his 650E. We were able to repair it by setting the metal tab again.

[chuckycheese]

Vacuum

If the petcock is operating properly, it takes very, very little suction to make the gas flow...

[duaneage]

This is true however it takes about 1 to 2 minutes to fill the carbs up and the battery might not be able to crank it over that long. Turns into catch 22. Fill the tank to increase fuel flow a bit, more gas will flow faster with a full tank.

There are little filters on each inlet valve of the carbs. If those screens are clogged it might cause a problem too.

[glenncarpenter]

It is also possible to have the vacuum line attached to the carb of a cylinder that is not generating vacuum. I had this happen once. If I recall correctly it was simply misadjusted valves: once I got them right the problem went completely away. Until then I had to run the bike with the petcock on prime.

[chuckycheese]

Fuel

This is true however it takes about 1 to 2 minutes to fill the carbs up and the battery might not be able to crank it over that long. Turns into catch 22. Fill the tank to increase fuel flow a bit, more gas will flow faster with a full tank.

There are little filters on each inlet valve of the carbs. If those screens are clogged it might cause a problem too.

Yeah, but he disconnected the gas line....so the carbs would never fill up. He's just trying to figure out why gas isn't flowing from the tank.

[duaneage]

If the petcock is dirty those screens are next on the list.

[GS110X506158]

My experience is that very little vacuum is needed, so the easy way to check is to suck the vacuum hose. Before replace the petcock check for leakage on vacuumhose between carb and petcock