A great little engine but has a bad design fault on the low speed mixture adjusting screw, the o ring seal is very slack which causes the screw to turn itself in/out at random during running which plays havoc on the running stability of the engine, trying to source an o ring seal that fits better is proving to be impossible.
The seating groove for the o ring should have been made a little deeper so that a more suitable o ring could have been fitted, alternatively the screw entrance hole should have had a chamfurred or tapered edge to prevent the seal from being shaved off during installation.