l have a 503 in my Maxair drifter which has done 90 hrs since rebuild on certain days l lose 800 rpms ,one it will run fine all day next time it has full power 6800 at start up about 1/2hr flight it loses power back to 6000 rpm then other days that's all l can get out of it from start up ,put new plugs ,changed air cleaners check exhaust for cracks, both mags work ,clean fuel and filter ,even changed taco ,any body got any suggestions other than use it for boat anchor.
cheers Geoff.