I have a Polini Thor 2oo Evo engine. With a Bing carburettor fitted. The engine has always had a little problem, engine runs fine from tick over all the way up to 4900 rpm and from 6100 and higher. However the engine runs rough between these two power settings. If I make the smallest throttle movement the revs will run rough and if I keep making small increases in power I get to a point where the power band kick in and it jumps up to 6000 rpm and runs sweet as a nut. unfortunately for me at 6100rpm The aircraft wants to climb and I can’t maintain a level flight and 4900 the aircraft will descend. Any suggestions would be much appreciated