I recently finished my first kit (Sig Mid-Star) and I put an OS .52 four stroke on it. This is my second plane, my first being the Avistar RTF with the OS .40 two stroke. I was taught that a good way to check the need valve adjustment was to pinch the fuel line at full throttle and the engine RPM should temporarily increase. I used this technique for the two stroke, but on the four stroke the engine always dies immediately. I can richen the fuel mixture just short of stalling the plane and it will still die when I pinch the fuel line. So my question is,

Does the pinch test work for four stroke engines?


Tom Pruitt

Not very well Tom. The best way on a 4-stroke is with a tach. Lean it at wide open till you reach max RPM then richen it till it looses
300-400 RPM. A $30 tach is cheap insurance for a several hundred dollar engine.

David

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-----

Thanks for the responses guys. I already set the needle valve as David had said, but it was a bit disturbing to pinch the fuel line and not get what I expected. I guess things are OK as they are. Thanks again.


Tom

The pinch test is for two stroke engines. you will need a tach for a 4 stroke!