4-stroke setup problem
Here's the setup:
-New OS FS95 4-stroke, mounted in an equally new H9 Sbach.
-Engine is mounted horizontally with carburetor about same height as fuel level.
-Engine has never been opened up to check valve timing.
-New Type F plug.
-Byron 4-stroke fuel
-Stock muffler, with pressure tap connected to tank
Engine starts easily at the default needle setting of 2.5 turns, and makes a reasonable amount of smoke.
After warming up, engine responds normally to throttle advance up to about 2/3, whereupon it sags and will die if throttle is not reduced.
Behavior is the same as needle setting is varied from very lean to very rich. Idle jet adjustment affects nothing but the idle.
Changing plug had no effect.
Pressure tests on fuel system showed no leaks or flow restrictions.
Most mysteriously, the sagging problem disappears almost completely if the pressure tap is disconnected, as if the muffler is providing too much pressure.
Suggestions?