Same thing - rpm drop but the sound is kind of different. When I lean the engine rpm just drops without changing the sound. When I richen up the engine starts inserting 4-cycle strokes. Rpm of course drops, but the engine does not run as smooth as if it is to lean. What I heared in the air was more like a "lean sound".

The carb is mounted that way, that the regulator hole is facing backwards (the firewall). I wonder what kind of effect the moving air might gave on the preasure measured in there. Look at the picture below.

On the good note my muffler is holding up. This time I used welding rods I bought at Harbor Freight Tools. First I cleanded the muffler well with dremel (sanding drum) and brake pads cleaner. Then I wrapped the muffler in glass cloth to prevent from heat to escape and mounted in vise I used standart butane torch. I was able to heat it to the rod's melting point. Working with the rod for a couple minutes I achieved wery good spread and looks like a solid bond. So far I had 5 flights and the muffler is holding up (on the picture the pipe on the right side).