MOST 4-STROKE ENGINES I HAVE ARE ON AIPLANES THAT HAVE COWLINGS SO YOU CANT USE THAT METHOD. WHAT I DO IS NEEDLE THE HIGH SPEED NEEDLE IN UNTILL THE RPM STARTS TO GO DOWN SLIGHTLY THEN RICHEN IT UNTIL IT DOES THE SAME, THIS GIVES ME A WAY TO FIND SOMEWHAT THE MIDDLE. THEN I WILL USE MY TACH TO ACHIEVE MY HIGHEST RPM. AT THIS POINT I RICHEN THE NEEDLE BY 3-4 CLICKS( BECAUSE IT WILL LEAN OUT SOMEWHAT AT ALTITUDE)--- MOST 4-STROKES WHEN RUNNING PROPERLY WILL HAVE A SLIGHT SMOKE TRAIL ESPECIALLY ON EXCELERATION. HOPE THIS HELPED--- HAPPY FLIING-----