I am convinced. Carburetors are a product of satan. Every bike I've ever owned has had some sort of carburetor demon.

The latest symptom, I've been dicking with the Bandit trying to get the carbs re-tuned after moving down in elevation. Currently its getting about 20 mpg, running on 3, really stumbling bad under 20% throttle. I've got the smallest jets mikuni makes in there, the needles are at full lean, 3 out on the pilot screws. The only lead I have is the #2 pilot screw. When I run it in, as if to seat it then readjust, gas pours around the screw head and into the screw recess. WTF. I checked the screw itself, the tip isn't broken off and the O-ring on it looks fine. Float height is dead on 15mm. I pulled it back apart tonight and cleaned all the little passageways I could find. The bike has never sat for more than a month or two at a time and I have an inline filter, so crap in the carbs is really unlikely. Any ideas from the thinktank?

Carbs are Mikuni BST36's, if that helps anybody.

Any holes in the diaphrams? Pinched diaphram? Vacuum leak anywhere?
Just a few things to check, since you've had the needles out.

I hate guessing knowing it's just going to create more confusion. A rebuilt carb don't always mean the it's fixed.
I am not familiar with that particular model of carb, but they usually regulate fuel or air and when working the fuel should be drawn to the low pressure of the venturi. That passage could be blocked or worn affecting overall jetting for that cylinder. Apparently this one is fuel.
Another problem I have encountered is the floats being set properly could still have a slight pin hole causing higher float level or perhaps not shutting off the fuel at all. The next thing is to make sure the needle and seat aren't the problem. Definitely check the diaphragms on a CV carb. (my new Indian has the HSR 45 I think)
Once you have inspected/equally adjusted the carbs, as far as possible at least by using factory settings for the time being, then I would start looking at electronics/electrical. It doesn't sound like all the cylinders are acting evenly, so check plug wires, plugs for color and gap, and connections to the coils or ignition components. The easiest stuff first. A quick check of the valve settings to make sure none are too tight, which can happen as the valve face wears into the seat.
Another really easy check is fresh gas. Dump the old stuff and put in fresh after, or before you go to all this other trouble. I bought fresh gas in a can from one of the local stations and it was BAD! I couldn't believe it. They denied any problems, but most folks driving cars filter out the water or contaminants. I had trouble with their diesel once as well. They just don't sell the volume other places do.
It sounds like you had to reduce the size of the jets pretty radically, which is unusual. A couple of sizes and an adjustment of the needles for mid range. See if a local dealer will offer standard jetting for that area so you have a baseline to work from.
Didn't mean to babble, but usually it's something simple. My personal experience is to always start with an
engine rebuild and work my way backwards! HA! Hope it helps some. Good luck.

Thanks guys, I'm piddling with them now. It idles pretty well, so that has me thinking something other than a vaccum leak, but I will verify it for sure when i stick it back together. I double checked the diaphragms, good to go, I forgot about float buoyancy, I'll check, although the plastic ones aren't nearly as bad about getting holes as the old brass ones. I really think the gas coming up around the pilot screw head has something to do with it, I just don't really know *how* they do their thing, so I'm not sure how to diagnose it.

I want to sell this thing and get a motard, but if i can't fix it I can't sell it, and if I could fix it I wouldn't be so inclined to sell it...

You could try to find a diagram of the carb, or information online about that particular model. Also you might be able to get something from Sudco International in Southern California. They are the Mikuni Distributors for the US, or at least the west coast. They might be able to e-mail you a flow chart of the passages.
I would root around in my old stuff, but I think my newest book is probably 20 years old. Probably doesn't have your model in it. LOL

Ha, well I appreciate the thought. I played a similar game with my old GS750, which had the old VM slide carbs...talk about a nightmare to tune....

If you haven't sorted it out yet I would suggest you check the needle and seat. I have been doing carb work for 35 years and I have found that problems like you describe start there. Is the seat tight in the carb, does the needle have any sign of a ring imprinted on it. If so it's shot! Make sure the tang on the float pushes squarely on the needle. Uneven pressure on the needle will cause it to leak or wear prematurely. Sounds like you have the rest in hand though you may find that when the fourth carb stops flooding you have to go up in jet size on all the carbs. Good Luck!

How do you check for a vacuum leak?

There are several ways but using a fluid around the intake tract in several places will eliminate leaks between carb body and cylinder. Spraying fluid (I think they make something for that purpose now. we used to use starting fluid. Not a good idea because it is so flammable) around gasket areas, rubber mounts to carb, etc.

The diaphragms would have to be removed to check for pin holes holding them up to the light or spraying with fluid to see if it migrates to the opposite side.
Generally the engine rpm will increase/decrease slightly at slow idle.

Depending on the type and year of the machine, they had vacuum gauge ports in the intake manifolds to hook up test equipment. I used them for balancing multi carb engines, but comparitive readings will let you know if there is a leak, or at least a differential. If you have those type portsm make sure the rubber or plastic caps are in good condition.
They make another inexpensive tool to lay over the velocity stack of carb(between air cleaner and carb), that youcan use to check idle opening and volume of air flow. The absolutely must be equal to run correctly or you will get very poor. This could help to isolate the problem area any way.