I'm using a ST 90 on a Top Flight Corsair 60 with a Bison Muffler.
The stock muffler is not a tuned pipe but as stock mufflers go is fairly quiet. I'm using the Bison muffler only because it fits in the cowl. The low speed needle is a fuel metering needle. Turn it clockwise to lean, counterclockwise to richen. Set the high speed needle first. Lean to max RPM then richen 1/8 to 1/4 turn. Then set lean out the low end until repsonse is good and idles reliably.
Most people leave the low end much too rich. At this setting the engine runs great with good reponse. You may have to richen the low end just a tad to make it easier to start. I have run it with a 14-8 APC two blade and a Master Airscrew 13-8 three blade. Runs good on both.