I have an XYZ 50 engine in my small Ziroli P47. The engine has good power but it is giving me a problem that I just cant seem to solve. Engine runs fine on ground, with good transition. Engine runs fine during take off roll. Once the plane gets in the air, the engine sags for about 1-2 seconds then comes back up to power. I thought it might be a pressure problem, so I soldered a tube to the diafram plate and extended it into the fuselage. This had no effect. This situation is giving me heart problems and it is only a matter of time before it will cause a serious problem.

It is strange that it is only exibiting this behavior when the plane leaves the ground it occurs about 4-5 seconds after the plane takes off. After the power comes back up it seems to run just fine. During todays flight it seemed to sputter a little during the first victory roll. I have replaced all the fuel lines and the tank is tight. Any help would be appreciated.