Try looking up the exhaust pipe with a flashlight or push a wooden probe up there to check for blockage.
The diffuser baffles (cones) inside the muff can break off due to fatigue/heat and block the top of the exhaust pipe. The result is a smooth running and normal sounding engine with a very reduced power output. This type of blockage can be intermittent as the lose pieces move around and may not happen on every flight.
It is a known but somewhat rare problem on G/A aircraft.
Also check the fuel tank vents are not blocked and check the mixture control cable for condition and attachment at both ends. Ensure that the mixture control arm on the engine can not move on its own (free-play) due to wear or damage or disconnection.
Last edited by Tree; 2nd Feb 2003 at 22:41.