I have a 1985 F150. The rebuilt 302 engine, I think is about a 1979 model and has about 40,000 miles on it. Until recently, it hasn't been driven much. Ever since this engine was rebuilt, it seems to have a slight missing to it and gets about 9 miles per gallon. I have replaced and checked everything I can think of. It is very cold natured, takes about 5 to 10 minuets idling to warm up before it will move. Once it is warmed up, it idles smooth. If you hammer down on the pedal, it runs good. The harder you run it, the better. But at normal driving, it begins to miss or hesitate, just slightly, just noticeable. It is a 4 speed standard short body. Doesn't use any oil. I have replaced, plugs, wires, distributor cap, Rotor, fuel filter, Air filter, PCV valve and can't seem to find a vaccum leak. Vaccum leak? Could this be it? Does anyone have a good method for vaccum leak testing? DOes this sound like the culprit? I have tried about everything I can think of. Any ideas? I need help. I think this truck originally came with a 6 cyl engine because it is geared rather low. Maybe that explains part of the poor gas mileage but still doesn't answer the annoying engine missing. What else could it be?
I also noticed oil in the breather cover, maybe three table spoons full, not sure what that means.

Any suggestions appreciated;
Thanks.
Flip;