That OS 46AX is a really reliable engine, Ive swapped it between 3 different planes and never had any problems starting or tuning. I currently have it on my Twist 40 with a 12x4 prop and I get about 12000 RPM's. When I had it on my Sig Four Star I was running a 11x6 and that plane flew really fast, a little too fast, I learned the hard way about over-stressing a plywood/balsa frame assembly. I snapped the right wing panel off after a high speed dive then a quick climb out. Needless to say I have learned throttle management and plane limitations from that situation. But I also agree with the other guys on trying different props, it makes a world of a difference. The higher the pitch the faster it will go, but the only down side on the higher pitch props is once you point that nose straight up dont expect a whole lot of climb out.