A hot muffler is normal.
If it pops and sputters when you advance the throttle, it sounds rich.
Strap the plane down and carefully adjust the main needle at full throttle until the RPMs peak being careful not to lean it (clockwise) once peak is reached. In other words, once the revs start to fall off as you turn the needle in, quickly turn it back out so you don't overheat it. Now, once the peak is found, adjust the needle out (rich/counter-clockwise) three clicks. This should be a good starting point for the top end and keep it from sputtering.

Kirk