Ldalmas the STs are noted for needing break in time put some more fuel thru it before worrying about the muffler the revs will come up martin
that big stock muffler is fine as a ringed engine it will be 500rpm less than ABC 50 just a charateristic of ringed engines for sport and pattern they are prefered, on rpms with time it will come up
the 51 as after thought may be ABC but still needs more running martn
the timing on the STs is such they develop their power at a lower rpm than OS so prop to the engine looking at Hobbsies post it turned a 12x7 at 9650