General aviation pilot reported a partial engine failure during cruise and returned to departure airport for normal landing.

Narrative

Was asked to test fly and do initial engine break in for a M20K with a 'Rocket' conversion. (Continental TSIO-520-NB). After preflight and run up accomplished a normal take off with no abnormalities noted. Tower asked that I switch to ZZZ approach for monitoring above the Class D. Per request of maintenance we orbited the field at 75% power. Maintenance requested we vary power between 65 and 75%. After 15 minutes slowly pulled power back to 65%. On advancing power to 70% an slight stumble was noted in the engine output. Advanced power to 75%. Sudden change of engine noise and noted manifold pressure reduced to 25 inches. Attempted to advance throttle; no change. Advised ZZZ we had a partial engine failure and needed to return to the field ASAP. They had us switch to tower. advised tower. When they asked to maneuver for a normal pattern. We indicate we would prefer to land immediately and requested priority handling. Engine continued to run normally at partial power and normal landing was accomplished. Take away: first flight was after major (engine overhaul) maintenance. Slow and methodical planning. All pertinent data for the Rocket conversion was reviewed. A plan was developed (what if the engine doesn't make power; quits; etc). After a smooth 35 minutes of normal ops; the abnormal developed. Having a plan; in advance; made this incident more of a none event.