This is how I solved the pressure problem that was created due to insufficient backpressure in the tank because of the pitts style muffler. So I simply closed one pipe with red silicone and added an additional stop pipe. I am now fully satisfied how the engine runs even without nitro.
What a beautiful day in Skopje. Shadows clear as if it were June, not March. Great Visibility. I was trying to get the engine to run smoothly, but was not fully satisfied. Will try something else tomorrow before the second flight. I begin to think that the pitts style muffler is not producing enough pressure for the engine to run rich so I'll try closing one pipe.