I have a Saito 40 on an ARF Cardinal.  It ran perfectly last year with the following setup:  APC 12 1/4 X 3 3/4 prop, pressurised clunk tank mounted on center with the engine.  This was one of the best combinations I have ever used.  This year starting with the first flight it runs fine right side up, inverted the engine slows by at least 15-20%.  So far I've changed props (11 X 5 and 10 X 5) ,put on a wedge tank (and moved it up and down)  and taken off the pressure.  It runs exactly the same in every case.  It seems to perform better once I get below 1/2 a tank of fuel.  I'm on the verge of replacing the Saito with an OS 46.

Any suggestions would be greatly appreciated.

Sam