Cylinder #2 Not Firing with Vacuum line plugged

[Guest]

Hey guys, I've got the ol' 1982 gs750T set up so the jets and needles are about as close as possible without a dynojet kit, but I've got a problem with cylinder #2 firing sporadically or not at all. It seems that if the vacuum line to the petcock is left unplugged and open the cylinder runs nearly perfectly but as soon as its attached to the gas tank, or plugged with a plug the cylinder boggs and drowns out... I've been working on this with advice from my motorcycle mechanic buddy but he's seemed to be a bit busy with other things at the moment so... JUST curious if anybody here has had this problem and solved it, or if there's maybe (obviously) something I'm missing.

[JJ]

Sounds like the petcock is letting gas go through the vacuum tube into your #2 carburetor. Time for a new petcock. You can try to rebuild it, but that doesn't have such a high success rate.

[tkent02]

He said it does it with the line plugged too, it's not the petcock diaphragm.

[JJ]

I missed that part... Thanks

[gsrick]

Could the enricher needle be open on your #2 carb adding too much fuel? The vacuum line maybe allowing the extra air it needs to run.

[Almonus]

thought about that but after inspection it seems to be set the exact same as all the other needles. I found that when I removed the plug from the vacuum line I also turned off the choke and the cylinder fired up. Now when I did both it seemed to run great (although pretty rich if the exhaust smoke is any indication) would a problem with the choke system cause these issues?
Basically I've checked all jets, needle heights and main passageways multiple times to see that they move fuel freely, as well as having just replaced a faulty needle valve thinking that was main the issue... which apparently it wasn't.

edit: Wait, are the enricher needles the ones that meter the mixture, the main jet or the float bowl?

[gsrick]

Choke by common name, but it's not really a choke. If the plunger is worn out inside it may be allowing fuel to be drawn though that passage way.

[Almonus]

Awesome! That's the next thing to check then. I'll probably take it apart and see how it is before this weekend and hopefully get it back up and running.

[gsrick]

The enricher is like a mini carb inside the carb. When you pull the choke lever, it opens an passage that allows more fuel for cold starting. If the plunger is bad then it may be allowing that extra fuel all the time, so with the vacuum hose open it might be providing the extra air to compensate for the over rich A/F mixture.

[Almonus]

Uhh... Pilot Jet fell out?...

So as I was disassembling the carbs to inspect the choke plungers... I found the problem... the pilot fuel jet seemed to have not been properly tightened and managed to back its way out. If it weren't for the little rubber stopper it would have fallen out into the carb. Reseated it, tried it out and all cylinders are firing now!
To think that all the times I checked to make sure there was proper flow I never bothered to make sure the jet was seated. I'm kicking myself now! Well that was a stupid waste of a few good riding months, now all that has to be done is a proper carb balancing and re-installing the tank. Thanks for the help guys, maybe I can get to riding again!