Hi all, thanks for the feedback.
Finally I had the chance to fly without muffler pressure. It was amazing. Easy starting, steady run from start to end. Very easy to needle. Not critical at all.
In flight the engine appeared to "understand" the load conditions much better.
So I can stop worrying about the issues with muffler pressure. Without muffler pressure, the engine behaves much better.
I really like the fast 4 stroke and the extra power were needed.
I also sealed the Tank compartment better against the engine heath. Perhaps it helped, perhaps not.
Now main focus on more trimming, the shapes, the bottoms and the sizes...
Thanks again.