I have been testing the engine with various props this weekend after carb rebuild including 24x8, 23x10 and best I could get was 5500 rpm peak with a 22x10. I don't think anymore that the problem is on the carburetor. I have leaned it as much as I can to get this rpm and the engine was hot and spray from the carb was minimum. It did not fail due to heat but not sure would be trusting to fly with those needle settings. Someone in the club suggested that the muffler I have (Krumscheid banana) is quite restrictive, so, it is normal to get bad performance. I loosened the muffler (not removed fully) and I could get few hundred rpm more. I am open to the suggestions. I have last week bought another of the same engine second hand for a very reasonable price, so I will very likely swap the engines also ignition modules to see whether there will be any improvement. I am open to suggestions. I already got the suggestion "get a DLE 55" but putting in any other engine will require major surgery and this is a very rare plane, I really want to make it work as it is..