The carb is leaking air. You need to get it seated down in the crankcase tightly or it will leak air and never shut down.

Loosen the nut that holds the carb on. Take the carb off. Put a little engine oil or castor oil on the carb where it slides down inside the crankcase. Now put the carb back on the engine. Grip the front of the engine with your hand and put your thumb over the top of the carb. Squeeze it down on the motor to compress the O-rings and seal it up good. Now tighten the bolt to hold it in place.

You can also use Flyboy Daves method of putting a small block of wood on top of the carb and tap it down with a hammer. Works fine.

You just need to get that carb down in there TIGHT and get it seated on the O-ring or it will never seal.

I run most of my TH .75 engines with a Pro Zinger 13-5 prop. It's the perfect prop for 3D on a 5-7 pound airframe. They scream. Can't remember the RPMs. Haven't tached one in over a year. No reason too.

I wouldn't run more than a 13-6 prop on this engine. Even a 13-6 is pushing it. Too much load leads to heat build up and causes deadsticks.

The Master Airscrew 12-8 is an okay prop for sport flying.

Just depends on what you want to do with the engine.

On the muffler attachment -- just cut 1/4" (6mm) off the allen wrench. Then it fits in the tight spot and makes it easier to attach the muffler. I don't like the muffler attachment either. Small price to pay for such a nice running engine though.