Just rebuilt a Stihl MS170. I did an ultrasonic cleaning on the carb block and rebuilt with a kit (metering lever flush). It was very hard to start, and I had to do some judicious bending of the choke rod so that the choke would close completely (thanks to an archived post). Although I couldn't find anything in the service manual that was relevant, somewhere I read that the initial adjustment on that single left hand thread blunt screw that does who knows what is 2.5 turns clockwise. Tried this and it started right up but idled too high, around 4500 rpm (no bar or chain). Fiddled with this and found that by turning counterclockwise (backwards), I could get the idle down around 3200. Idle was pretty steady considering the engine had new rings, but higher rpm's were a problem. I had to feather it a lot to get to around 9000 (bad flat spot in the acceleration) and as i gradually went to WOT it suddenly screamed to 12000 and I immediately backed off. Adjusting the screw did not solve this problem. The manifold does have the plastic ring and not a clamp, but I put Dirko in the outer groove of the cylinder intake port so the manifold would have a little help sealing. It sounds to me like it's running lean, not getting enough fuel. I'm suspicious of those closed carb passageways that can't be cleaned, and wonder, even with US, if they came clean. The plan is to replace the carb with an adjustable one and see if I can get a good idle and smooth transition to high rpm's. The question is, does it need more breaking in (wood cutting) before deciding what to do, or is this taking a chance on burning up the piston.