Quote Originally Posted by mushroom_toy View Post
Actually mains can have something to do with it. Until we know whats done to the bike or how its tuned we dont know for sure. If the main was completely clogged, it could affect idle as there is some bleed through at the needle (which the main jet feeds). If the bike were tuned taking this bleed through into account, and the jet was then clogged, the bike would not idle.

And if you read what I said more carefully you would understand that I didnt say that the mains specifically had anything to do with idle. Read it again. I was giving a scenario where the mains were too rich and the pilots too lean according to the mods possibly by the shop or someone else. I didnt say if the MAIN is jetted too rich only.
I've seen several accounts of bikes with mains being completely stopped where bikes would idle fine, and as soon as you try to move off idle they die. The ONLY thing affecting idle directly is the pilot, the needle begins affecting as soon as the throttle is touched. Until then if properly setup the pilot is the only thing affecting idle. The overlap between pilot and needle is not enough to necessarily cause any issue at all with idle. Also seeing as how he said it ran great for a month I doubt the main would be the problem, mains are the hardest to stop up due to of course the biggest hole, and if the mains where incorrect the bike would have never ran great to begin with.

I used to deal with carb and fuel injection tuning on a daily basis, everything from Ironhead Harley's, to a low 8 second N/A ZX14. If his mains where too big (or small for that matter) he would have noticed the bike not running well. If it's idling for a few seconds it really seems like the issue may be in the petcock or trash in the tank stopping up the petcock as you said. Also if it's a vacuum operated petcock it could be getting insufficient vacuum from the case causing it to not pull enough fuel through. I know he replaced the lines, but that doesn't mean that it is necessarily pulling enough vacuum. I'd pull the carbs, check pilots, check float level, clean everything else while you have it apart, then bypass the tank line by putting a feed line directly from a fuel source (We have a bottle with a nipple tip running to a regular fuel line to put onto the carb) to bypass the tank/petcock issue all together to see if you can pinpoint the problem.