RE: tower 46 troubles
This is the same problem that the GMS47 engines suffer from and is indeed related to a lack of muffler pressure.
In our club we've solved it by fitting a different muffler (TT46 or Tower Hobbies) or fitting a restrictor to the outlet of the GMS muffler (and welding the two halves together because they always leak and end up falling apart anyway.
Whether the lack of muffler pressure is simply a symptom of bad tollerances in the carby is uncertain -- some have reported that drilling out the fuel-fittings on their engines have achieved the same result.
What we do know is that fitting a small restrictor is very simple, reversable and does the job just fine.