Mixture screw leaking

[Kentish]

Some of you may remember my last thread about a petrol smell. Well today I found that it was still there so I decided to put my finger under each mixture screw hole. I found that cylinder 2 was weepin petrol. This was also the cylinder that was low on vacuum when I balanced my carbs. The thread seems fine as the screw turns in and out easy. Has anyone else had this problem? Could it just be an o-ring? I can't understand why it would leek though.