Need help inconsistent idle, vacuum leak etc etc. 92 B4

[ED]

Ok, I have been trying to solve this problem on my 400 for quite some time.  So here are the particulars

It's a 1992 Bandit 400 with 25K kilometers on it.

The engine was just torn down, and the cam chain replaced, while I did that, I set the valves, so they are correct.
I cleaned up the carbs while I had them off, but I basically left them attached to the rails and just pulled the bowls and bellows caps off.  All

the vacuum tap o-rings are accounted for.  Carbs were synced after the engine was put back in.  I checked the petcock and vacuum hose with a

vacuum gun and it holds vacuum no problem. 

The carbs/bike is bone stock. No performance upgrades or carb additions at all.  I have the A/F mixture screws set to 1.5 turns out.

The symptoms...
When the bike is cold, it runs beautifully.  Pulls really strong, no hesitation, but the idle will hang a little at 2K rpm

As the bike warms up, the idle becomes more erratic.  It starts to hunt. The bike will be really slow to return to idle and hang at 2.5-3K for

many seconds. If I go out and ride, I will lose a cylinder fairly rapidly.  Sometimes it will come back, but usually only if I hold the throttle

wide open, and let the revs climb above 8K.

If I richen up the A/F mixture screws, the hunting is not as bad.  If I pull the sparkplugs, 1,2,3 are whitish, and 4 is black.  When I drained

the carbs, #4 had almost no fuel in it at all.

I have used every method in the book to find a vacuum leak between the carbs and the engine block.  WD40, water, carb cleaner, even smoke.

Ok now for the weird part.  I removed the airbox to expose the carbs.  With the bike running at idle, I place my thumb over the pilot air holes

at the mouth of the carb starting with 1.  For each carb, 1,2,3 the bike idle will drop off and the bike will almost stall.  When I get to number

4, and place my thumb over the pilot holes of this carb only, the idle rises to 2-3K, and will stay there more or less. 

I tried squirting WD40 on the seals that join the fuel rail together, it might have been my imagination, but it seemed to dampen the effect of

placing my thumb on the pilot air holes.  Is it possible that an air leak at this location could cause this problem?  I mean, it's full of fuel

right?  There shouldn't be any vacuum at this location. The other thing I was thinking was a stuck float.  If there was no fuel in the bowl,

could that cause these symptoms too?  Certainly the cylinder not firing, but the rest of it as well?

Any advice or help is much appreciated, I am pretty close to dropping some dynamite in the tank and wishing it a fond fairwell.

ED

[andrewsw]

welcome to the joys of B4 carbs. When they're spot on, it's awesome. Otherwise... not so much

How bizarre that you'd have no fuel in #4 but a black plug...

The way it is behaving sounds like a typical stuck float valve/needle. What happens is the thing will stick open and flood the carb. High revs will help burn it off and it will run because the fuel demand is keeping up with the over flow. As soon as the revs come down again, the darn thing will start to fill up and flood out again.

Pull the float bowls, pull the floats and investigate the needle valves. They should move freely in their tube. Sometimes you have to actually delicately pry the damn things apart to get the gunk out of them... And while you're at it, make sure the float levels are all good.

If the idle is hanging, that's surely leanness on the a/f mixture screws, but don't mess with that until you get the other stuff sorted out. It shouldn't be causing you much trouble anywhere but on or right near idle.