You are using a prop which is too small for that engine. 3W-24s shine with a 18 x 8 for most rc applications. In sleek aircraft, an 18 x 10 is perferred. You should get around 8000 rpms with an 18 x 8 with the pitts muffler.

If your plane is 12 lbs or less, that engine should give unlimited vertical. I found the best prop for the engine was actually the Master airscrew classic 18 x 8. I tested a bunch of them when I used that engine on research planes. At one pt, I had 18 of them and it is a great engine.

Elson