Thanks for the input, the throttle servo is stable and although I am by not familiar with this make of engine, the compression is incredibly high compared to my other standard 90's, SC's, OS etc, so I can't understand how it can be a piston blow past problem. I have also inspected the piston liner for black marks which might indicate gasses getting past the piston ring and the bore is prisctine and has still got the original manufacturers honing marks. Thanks for any further ideas you fellas might have.