While my Y-S engine had the smoothest idle, fastest transition and greatest power of any four stroke that size I had ever owned . . . .

There is a tremendous design philosophy difference between a Y-S engine and a OS, Saito, or Magnum engine. People that say that there isn't a difference, are not completely familiar with the internals of a Y-S engine.

First of all the other brands do not have a sealed crankcase.

The Y-S engines have an extra seal in the front of the crankcase behind the front bearing. Also the carb(s) has a seal between them (or rather the two halves). There is also the requirement of an external check valve, an air tight fuel tank, and a myriad of tubing running here and there to make the fuel system run correctly. An air leak or malfunction of any of these will cause the engine to be erratic. In the regulator mentioned there is also a piddle valve that can stick or wear out in addition to the thin diaphragm mentioned. One needs a complete understanding of the whole system to be able to properly trouble shoot it. The "S" versions have a valve on the intake pushrod housing.

In addition, the 120AC and 91/110 engines have an air chamber on the rear of the cylinder with a thin paper gasket that is very touchy to get assembled without the bottom side raising up away from the seal area.

Even the rocker cover gasket seal is critical on a Y-S.

The engine and fuel system must be air tight.