I have a ST 40 that I can not get tuned right. It wont idle up to full rpms. I adjusted the high speed needle and thats not it. the low speed is very close. but the fuel mixture screw got messed with before I got the engine and I'm not sure how to reset it right. Could that be my problem with the rpms. You can hear that the engine just isn't hitting full rpms. Does anyone know how to adjust the mixture screw to get it back to where it should be? The manual says that the mixture screw should be set and shouldn't need to be adjusted, but dosn't say how just in case.