Spitting carb

[Trigger]

A little problem that i have never come across before. Finished re-building a set of 550 carbs. The bike starts great, runs perfect but, when you turn it off, number 4 carb will spit out a teaspoon of fuel out of the overflow pipe after 2 minutes. Doesn't matter if the fuel tap is on or off and does not do it every time you switch the ignition off. Any clues

[Trigger]

Float height OK. Does it on both stands and when coming back from a test ride. It is like a spit not leakage. It is like back pressure type of thing. Weirdest thing i have ever seen.

[Bryanj]

You sure there is not a small split in the brass overflow tube?

[Trigger]

Cheers for the tip Bryan but, stripped them back down and found nothing. Back on the bike, Number 4 stopped Spitting and number 3 does it now. Rode it round a couple of local shops and no one has see anything like it. Head scratching time.
It is also having a problem on tick over now. Set it up, ticks over OK. Then it will start to rev up. Re-set the tick over, then it stalls due to drop in revs.