You must have an in cowl muffler -of some type -
don't worry about rich sound in mid range - it means nothing .
good throttle response is everything
set idle screw IN till the engine sags -then oen a tad - try to rev it - open needle a bit if it tries to kill.
it is a trial and error adjustment -but if all is OK with carb, the low needle will be about a turn and a 1/2 or so open from seated.
The engine really likes big can mufflers or tuned setups and will run that prop way up into the 7000 range on a pipe .
As mentioned -- DO play with the needles -turn em -listen run up the engine listen again-
Only you can be the expert on your engine - and it ain't hard to learn -just do like your school teachers commanded - LISTEN!