Metal arm, idle screw slightly below flush. I set idle using the pinch test and it seems about right. No setting of mixture eliminates or reduces the problem. It sounds like, especially at idle, a series of explosions going off, like I said BAM, BAM, BAM with vibration. I know that combustion engines are a series of explosions, but this thing is really noisy, not smooth sounding at all. I just don't have the experience to know what a rod knocking in rc engines sounds like, or if one can look at the rod bearing fit and tell if it is to loose.