Hi PA-18,Alright, so you applied choke and the motor ran better (don't worry about speed yet)That would indicate that you are running too lean. All the intake and carb screws are tight, you have new diaphragms (stiff metering diaphragms cause lots of problems) and gaskets in the carb, and you cleaned the inlet screen in the carb. You've verified that the fuel lines are not cracked, kinked, or plugged and fuel flows easily from the tank to the carb. The exhaust system is tight and so are all of the screws on the engine (jug, back plate, spark plug, etc. )Make sure that the crankshaft doesn't have much play in it (leaking bearing seals)I want you to work through the tuning on this link and report back.http://www.footflyer.com/fix/motor/carburetor/walbro.htmI'm thinking that the metering arm is out of adjustment, but work through the link above first.