RE: tuning problems on old ST 61series x rear exh.
Your pipe length is simply too short for the prop size you are running. Either go down in prop size or lengthen your pipe. This engine was designed to spin an 11x7 prop fairly fast as it was designed before the "slow & quiet" era of pattern flying. If you aren't getting the prop up to at least 14,000 rpm then your combination is off.