Help with 80' GS450 fueling problems

[gs 450 man]

Hey Guys/Gals!
I was given an 80 GS450 and need help getting it to run correctly. Initially I had a problem where the bike would start then when it was accelerated a little, fuel would start backing up into the airbox and left cylinder untill it died or I shut it down! It's all comming in through the left cylinder and the right seems to be working fine. I have pinched the vacuum tube going back to the petcock and it seems to eliminate the overfueling problem, but now the bike only runs for about 1-2 miles, then it slows down and dies (its not fun pushing it home)! I have cleaned the carbs out thouroghly, and everything seems to be in order, floats, jets, and needles. I really need help cause I need to use this bike to take my licence test! Thank you...

I have read on a few other threads that the petcock on this bike is opened by a vacuum diapram. Maybe thats my problem???

[daveo]

sounds like the same as this gentlemans' problem...

http://www.thegsresources.com/_forum/showthread.php?t=96619

[bgmart450]

I would guess that if you pinched the vaccum line and it stalled then your petcock isn't wholly shot.

I'd check your floats (as noted above) for height, being stuck or even cracked. Those brass floats, yech... I have had at least 2 of them that were cracked and taking on gas. I had gas pumping out the overflow when that happened.

[gs 450 man]

Well, I drained the fuel through the petcock and while applying vacuum to the vacuum side did open it fuel also exited through the vacuum side but at a much slower rate. I think this is where the left cylinder is getting all the extra fuel. I took the petcock off and opened it up to take a look at the diaphram and there are no aparent holes in it allowing fuel through, but it is kinda dry feeling, how do I fix this? Why does the petcock close anyway? Can I just modify it to stay open? It seems like the floats should close off fuel to the carbs anyway.
Oh and the floats and everything inside the Carbs checks out, I have checked them three times, and I had a friend check them as well.

[nabrams]

Your floats/needles/carbs are not at fault. If fuel is coming through the vacuum line then that carb is getting too much fuel. Get a petcock that does not require vacuum and plug that vacuum line to the carb so that it doesn't pull air or fuel - then all should be well.
And remember to shut the petcock when the bike is not running, just in case your float needles do leak in the future.