Rebuilt my 1978 GS750 Petcock, now runs lean

[G.S.Joe]

Hi all,

I've finally started re-assembling the bike after fixing a few things (none carb or engine related, this is relevant) and now the engine only runs with the choke 90-100% engaged. It runs at very high RPM, and when I open the throttle it dies immediately.

This is the same rebuilt kit I used: http://www.ebay.com/itm/K-L-FUEL-PET...408#vi-content

When I took apart my old petcock, there were 2 differences between the old and new. #1, the gasket between the petcock and the tank had an extra tab I had to trim off on the new one. #2, The large O-ring style gasket was seemingly non-existent in my old petcock and I did not see any obvious places to put it, so I omitted it during the rebuild assuming that it was a feature on the bike in another year.

In retrospect it seems like that gasket goes somewhere and it's omission is not allowing the diaphragm to open fully in the petcock. Any ideas if it should be on there, and if so where it goes?

Also, I checked my vacuum line; it's in great shape and seals nicely.

As always, thanks in advance guys.

[MOC]

there's a little hole in the diaphragm that needs to line up with a hole in the petcock body. did you line those up appropriately? If not, the vacuum of the engine won't be enough to suck the diaphragm open.

[terrylee]

If you haven't rebuilt your carbs. that probably should be done. Check out Basscliff's site for how to instructions. Obtain required o-rings from "cycle-o-rings". Also he will have o-rings that are between your carb. rubber holders and the engine. That can make your bike very lean if old and brittle. Be sure you airbox is sealed properly. The petcock could be part of the problem, but sounds more the things I mentioned. terrylee

[terrylee]

Also may need to adjust fuel and air screws. terrylee

[G.S.Joe]

Also may need to adjust fuel and air screws. terrylee

It ran and idled fine a month or two ago. Though the carbs are the next thing to get torn down (and I'm not ruling out them being gunked up from junk gas from sitting for weeks) I think adjusting those would be more of a work-around rather than fixing whatever is all of a sudden causing it to run lean, no?

[G.S.Joe]

there's a little hole in the diaphragm that needs to line up with a hole in the petcock body. did you line those up appropriately? If not, the vacuum of the engine won't be enough to suck the diaphragm open.

I did not! I didn't even notice it. I'll tear it apart tonight and report back. Thanks!

[G.S.Joe]

So, as suspected the holes in the diaphragm were not aligned. Got her running, and after warming up she idles decently without choke. That being said she also ****es gas through the overflow when I have it on reserve, so I'm still concerned that that large O-ring I didn't put in the petcock has a place and a reason. Any ideas?


EDIT: Just took another gander through BassCliff's site and he has a link to a petcock rebuild I forgot about. Yep, the O-ring goes behind the dial!