'80 GS750(E?) no start

[NastT]

I bought this bike at the end of last summer, it was mostly running well but had a few other electrical issues and had just started dripping a little gas from the air intake of the far right (#4?) carb. I took the tank off to sort the wiring and I replaced the petcock vacuum hose that I thought might have been the cause of the leaking fuel. When I put everything back together it cranks but won't start (choke in or out). I suspect it's not getting fuel. It's not dripping fuel now though. I'm wondering about the hose that attaches to the tank at the nipple in front of the petcock. Is that the tank vent? Does it attach to the carb somewhere? I'm 99% sure I have the fuel & vacuum hoses attached in the right place. I have a shop manual and looked online but can't seem to find info showing exactly where that hose is supposed to go. Thanks in advance for any suggestions! PFA

[TonyGee]

put it on PRI ( prime) and then try it. 

[NastT]

I did, but maybe I didn't let it turn over enough to circulate the fuel. 

[Blubber]

The nipple at the back of the petcock operates the valve by vacuum. 

The vacuum is delivered by the running engine and is usually taken of a nipple fromnthe carburettor.

If you put the petcock on pri , you prime the carbs : fill them by hand. 

If the PRI does not fill the carburettor some hoses are routed differently than intended.

It kind of sounds like you have connected the filler cap drain to the fule delivery side of the petcock but hard to tell without pictures. 

 

[NastT]

There are two hoses that attach to the petcock. One is the fuel supply and the other is the vacuum, both attach to nipples on the carbs. There is a 3rd hose that that attaches to a spot under the fuel tank just in front of the petcock but I don't know where that hose should end up. Is it a drain? Is it a vent? I'll try to post some pics soon. I haven't been in a position to work on the bike and I'll need to take the tank off again.