When Y-S engines are in good shape there is absolutely no engine that runs better. They have perfect idles and make power like a engine a size larger than they actually are.

But one giving trouble is a nightmare. The screw on the regulator should be flush with the outer edge of the regulator body.

The regulator has a spring and a viton valve plunger in it as well as the diaphragm. There's no telling what was done in there.

Your engine most likely has the wrong needle valve in the carb. The carb is a air bleed type and the wrong high speed needle adjustment will also effect the idle.

If I remember correctly the AC version has a different needle valve than the FZ version. The FZ 110 is the current production model. The AC 91 has been around for a while and the carb is different than the FZ.

Enjoy,

Jim