Tillotson MD carburetor? I guess you know that the skinny tube going down through the top is the idle metering jet. And there is supposed to be a gasket under it. It is what determines the amount of fuel going in at idle. And they are sensitive to dirt clogging the tiny hole. The needle controls air, not fuel. Screwing it in cuts off the air, making it richer (like backwards). Normal opening is around 1/2-3/4 turn.