Adjusting to the RPMs that you are looking for by richening the needles is not a good idea. If you run your engine rich you will develop carbon build up which can seize your engine.
You could i suppose set your endpoints on the servo but youre defeating your purpose.

There are 2 ways to achieve what you want to achieve.
1 is with the exhaust. You could put some cans on it to keep the noise down.
2nd is to prop it right. I'm sure someone will chime in and point you in the right direction on prop selection.