I have a Spitfire with a Thunder Tiger 120 Pro. The engine is broken in with rich needle setting on 15% fuel with suggested 13 by 8 prop. At full power it ran for 10 seconds and then started losing rpms. After break-in I put an oversize prop (15 by 8) and it ran great but a little down on RPMs (I thought because the prop was to big). The manual says the engine should have a 14 by 8 prop with 75% methanol / 25% oil. That is why I bought the larger prop because the 15% fuel I am using. After seeing marginal thrust compared to the 13 by 8 break-in prop, I decided to trim the ends of the prop 1/4 inch on each blade. Now the engine loses power after ten seconds just like It did with the break-in prop. Must I use an oversize prop to keep the engine running smoothly? I still have not flown the plane and I am worried I will lose power if I go to full speed. Is it the fuel? Should I switch to 5% or 0% fuel? It seems the engine does not like high rpms, although it is rated to 15,000. Should all engines be able to run at full power at the test stand without losing power or do planes need airspeed to keep engiones going smooth? This is my first nitro plane. I am scarred I am going to lose power on my first flight!