Miklos,


Pitts type mufflers have a tendency to reduce the fuel pressure to significantly lower levels, compared with the standard muffler, with an exhaust pressurised fuel-tank.

It makes itself known when the engine leans out, as you raise the nose.

It has been suggested in past threads, to crimp, or to plug-up one of the Pitts muffler's down-pipes.
This usually increases the pressure sufficiently.

The 'nose straight-up' test must only be passed at full power.
This, because under no possible flight condition will the engine need to overcome gravity, to siphon fuel straight-up, when at idle.