I test the engine today with the new OS #8 plug. It helps starting the engine, but the problem is still the same.
I endup almost screw out the whole regulator. I worry that there is a pressure (air) leak in the regulator. As long as I have more than 1/4 throttle, the iddling won't stop at 90 degree position.
I check the O-ring, it seems still ok. I am thinking getting a new one,but I am not sure if that is the cause.
The engine is old and the O-ring is old.

Or,I was wrong that I shouldl lean the regulator ?

Appreciate your help!
Thanks

Mike