I have a 91FX and it's failing. The engine keeps quitting on me and has "dead sticked" my plane several times. It also is overheating.

The "quitting" happens at high throttle. When going to full throttle, after about 7 seconds, the motor just quits...as though a "light switch" was turned off. There is no warning. It just shuts off.

I have also noticed that even with the needle valve wide open, I can't get the motor to run rich...to the point of quitting. (I have adjusted both the idle screw and needle valve in testing this.)

Further, it appears the engine has overheated...to the point of damage! I pulled the head and found the sleeve had seized to the head! Since then, I have replaced the head and sleeve with a used one and have the very same problems (deadstick and overheating). Also, I have replaced the carb....same problems.

Just to let you know, I have the tank near the motor (approx 1.5 inches) so no pump should be needed. Further, there is no foam, bubbles, or kinks in any of the fuels lines. Several new (OS 8) glow plugs have been used....and I am using omega 15% fuel with a 14/6 prop. All fuel lines are new and the fuel line to the "clunk" has been replaced...just in case.

The plane I have this motor installed in uses a cowl. However, I am not at the point of having the cowl installed. So the engine has been run without a cowl (eliminating air flow issues as a cause of overheating ).

To eliminate the possibility of a "lean condition", I have pinched the fuel line and the engine will increase in rpm significantly.

I am at "wits end" with this motor. I have 4 other OS motors and have never had such a problem with them. I'm afraid I am going to "toast" this motor again. I am about to give up on this OS90 and look for a comparable motor.

What could be wrong?