I have found the source of the problem. It is the slimline pitts muffler. I put the Tatone muffler on my .61FX and the engine really came alive and I got 12,700rpm with the Master Airscrew 12x6. I then put the slimline muffler on my Thunder Tiger Pro .61 and it too lost appx 1,200 - 1,400rpm with the slimline muffler. I dont understand why the slimline has done this but in any respect I wont be using that muffler. I have tried to contact slimline about the muffler but their email is not working, so I assume I will have to phone them. Thanks again for all of your help. I am not an engineer but to me it just seems like the chamber on the slimline is just to small. I have enclosed pics of the muffler. The slimline is on the right.

Johnny Millwood