I concur!! the muffler sucks! The engine is incredible however. Mucho power, great idle, great transition. I have a Tower muffler and the bolt holes almost line up. Since my bolt into the cylinder threads are trashed-(another poor design)- I am going to slot the bolt holes in the case to fit the Tower muffler.
This 51 just blows away my TT46 in every aspect except muffler pains. My 4*40 and I miss the ST51.

Have fun, MikeB