I have one of these engines. A real powerhouse. Rated at 1.85 HP at 16000.High speed mix is normal start at 2-2.5 turns out . Low mix screw is the screw running in the slotted arm. Move 5 degrees at a time, in for lean out for rich . Big yellow paper says don.t touch screws on pump, send back to OS. This was a new engine!!. Engine tinkerer that I am. Idled too rich no matter where the idle screw was. Turned the lower pump screw 1/2 turn.ran fine ever since. Very reliable, powerful engine. Questions? Just ask. Glad to help.