What was found
PILOT REPORTED RIGHT ENGINE FLUCTUATED IN-FLIGHT. WHEN PROPELLER LEVER WAS MOVED, RPM DECREASED. TROUBLESHOT SYSTEM. ADJUSTED IDLE MIXTURE AND SPEED. ADJUSTED FOR 30 RPM RISE AND 600 RPM IDLE PER MM. GROUND CHECK OK. UPON FURTHER INVESTIGATION, FOUND LOW FUEL PRESSURE PUMPS IN OFF POSITION. THIS MODEL REQUIRES PUMPS TO BE ON DURING FLIGHT. TEST FLEW AIRCRAFT FOR 2 HOURS WITH NO PROBLEMS. AIRCRAFT RETURNED BACK TO SERVICE.