The inlet manifolds can leak if the bolts are over tightened. You have to be quite carful.

Andys 21 used to be very bad at tickover, but his crank was shot and so were his seals. Its pretty much perfect now.

The only other cause coudl be too weak on the pilot air screw or maybe a size to weak on the pilot jet.
_________________
If I have to take the carbs off once more...