My YS 91 will not run at full throttle when it has warmed up. The engine keeps surging at full throttle (pulsing) even though it is running slightly rich. I have been informed that this engine can suffer from a bad seal at the supercharger plenum chamber or a cracked chamber. I have lapped in the plenum seal face down to a few microns and replaced the gasket, and the chamber is not cracked, the engine still will not maintain full throttle. It sounds as if the throttle is being open and closed every second but the engine does not cut. The pressure in the tank maintains for at least a week so there is no leak either in the tubing or the check valve and I have checked the tube from the regulator to the carb. I have changed the plug and am using a proper YS 20% nitro fuel mix. The pressure in the tank at full throttle when it is pulsing is about 8 psi, no more so this indicates that there is no blow back past the piston. Any ideas?

Regards Tony