It's a Ryobi 31

You are correct about the needle. One towards the air inlet should have a "H" on it. It is your high speed needle.

One of the fuel nipples is a return line. Without one of those "Bulb" pumps in the return circuit, the nipple will suck air. You need to close it off with a piece of tygon fuel line and a plug.

The fuel nipple next to the needle valves is the return.

The muffler is not for a Ryobi. Bennett should have a muffler to fit? If not try Jag Engines or Carr Precision.