won't run with "on"

[goggleboy]

I just gave my 78' gs750 a thorough test ride and while doing so I noticed that it would run OKAY on prime, but when I turned it to the "on" position it would start to shut down. How can you tell if your petcock is the issue? I disassembled the petcock and didn't find any broken gaskets or o-rings, and the diaphragm is clean. Its frustrating given the fact that I already adjusted my valve clearances not even a month ago, I have new intake boots..I've already taken the carbs apart, the pilot and main jet are clean, haven't really checked anything else on the carbs... but I did adjust the air and fuel screw. I adjusted the float height, (I dont have the tool )
Any trouble shooting ideas?

[koolaid_kid]

The easiest way to test the petcock is to suck at it.
Seriously, with the petcock set to the ON position, connect a fuel line to the petcock fuel nipple, aim it toward a pan or something to catch the fuel.
Connect another line to the vacuum port on the petcock.
Suck on the vacuum line. If fuel comes out, the petcock is fine.
If fuel does not come out, switch it to PRI. If fuel starts to come out, it is your petcock.

[Steve]

... Suck on the vacuum line. If fuel comes out, the petcock is fine. ...

Of course, he means that fuel should come out the FUEL port.

You have to be careful sucking on the vaccum port, though, there is a possibility that there is a hole in the diaphragm,
which would allow fuel into the vacuum line. Gasoline does NOT taste good.

.

[koolaid_kid]

Of course, he means that fuel should come out the FUEL port.

You have to be careful sucking on the vaccum port, though, there is a possibility that there is a hole in the diaphragm,
which would allow fuel into the vacuum line. Gasoline does NOT taste good.

.

LMAO. When I was a kid, we had to siphon fuel for the lawn mower (and for my 150). Not only does it not taste good, it hangs around for a while.