Trash the Zinger Pro. They flex, vibrate, and on some the blades don't track. The vibration alone will keep the rpms from *****g out. You've probably also noticed a lot of prop noise. But also, every engine is different. My Poulan 46 is 1000 off some other peoples #'s. Last thing, is it too rich? Excessive rich setting does litttle to cool a gasser and just slows it down.