I have a 1993 f2. 22000 miles. I bought it after it had sat for 6 years, totally neglected. It was surprisingly easy to get it running but it leaks fuel from the right 2 carbs. At first I thought it was coming from the bowl drain but fuel appears to be coming from above that point and accumulating at the drain nipple. With the bike running, I think I can see fuel dripping from the bowl screw shown here.https://imgur.com/a/vPEBzkyI pulled the carbs, replaced the bowl gaskets, inspected the float needles and floats. No cracks on the float bowl. Everything looked good. Back together and on the bike. Still leaks. Pulled the carbs back off and this time set them at the approximate angle as when mounted and poured carb cleaner into the fuel inlet and it does not leak. I cannot reproduce the leak on the bench. So, I'm stuck and not sure where to go from here. Thanks!

 

It is probably the O-rings on the T joint between carbs 3 & 4, or the joint itself. They are known leakers. You might not see it unless there's pressure in the fuel line.

 

How difficult is replacing the o-rings? I have the rebuild kit but am a little intimidated with the prospect of pulling the carbs apart.

 

It's a little bit of a task. When you do this, you will need to remove the entire carb bank, then separate the carbs themselves. Anytime you separate them you need to resync all the carbs.
It's not a super difficult thing to do, but it does take patience and some special tools. Depending on the age of the bike and the last time the carbs were looked at, most people take the opportunity to do a complete disassemble, clean them and replace all the rubber components.
The good news is that that it is totally doable if you choose to do it and there are tons of videos to help and of course all the help on this forum.

 

From the sound of it, your needles/seats are leaking. When you manual fill the carbs on the bench, you're putting in a limited amount of fuel. So the carbs won't overflow.
When you put them back on the bike, they leak, just keep filling with more fuel and finally overflow.

Bench test them again, but use a fuel bottle hanging above the carbs. A pop bottle with a hose attached works. Just so you've got more than enough gas to fill the carbs.
You'll most likely find the leak right away.

 

I will replace the float needles today and see if that helps. Also, where is the fuel supposed to over from? Most float bowls I have seen have a vertical tube that drains fuel when it the level exceeds the tube. The f2 doesn't seem to have anything like this.

 

IIRC, the tube is internal, it bypasses the float bowl drain screw and comes out the bottom of the float bowl drain holes.

 

I tried your bench test method and it worked! So, I have fuel leaking from the jets (I think that's what they are) on the carb with the arrow and from the vent line. What does this tell you?
https://imgur.com/a/nlGt9Z5

 

about 99% sure that you have a float needle that is sticking or not sealing.
Float bowls are completely full if it's coming out the vents and air jets.
Its probably only in one carb that is leaking, try to figure out which one.
Problem is the vent connects all the carbs together, so by the time you see it overflow, it's filled the other carbs too.
I think you can check the right 2 and then the left 2 separately.

 

I'm in the sticky float valve camp. Clean your float valve seats with brasso and a q-tip, a much overlooked process that is definitely needed. If you replaced the seats (which I doubt), even when new they need to at least be cleaned with carb spray. You can test the float height with the carbs on the bike using a clear plastic tube connected to the float vent/drain nipple.

What usually happens after a carb cleaning is the bowls leak when the bike is not running and nothing happens while fired up. This is a more advanced stage of that lol.

And using ethanol free fuel & Sea Foam (or equivalent) will make you happy in spring.