weber problem

anyone had problems with the mixture screw coming loose?
1.8 clipper with newish weber running off the scale at 9.9 %
started last night, got home pulled plugs and they were as black as ghandi..
1 little slight turn of screw saw mixture go from 1.9 - to 9 % co..

 

does it have a rubber "O" ring on it?

 

under the screw? or on the outside?
put a little blue tack on outside try and stop it moving

 

as far as i know it should be around the screw it's self, about 1/4 of the way up from the bottom once removed.