It sounds like two issues to me, one is simply tuning and the other is the distance form the tank to the engine (not the height difference when the plane is horizontal).


i) If the engine speeds up when you lower the nose (from horizontal) it means that the engine is set/tuned too lean.


ii) If the tank is mounted far from the engine, then the height difference when the plane is tilted up will be too large. You need too mount the tank closer to the engine and/or increase the pressure from the muffler (using an OS muffler with baffle for instance).