I have a high mileage F150 2 wheel drive with the 4.9 Liter straight-6 cylinder engine that seems to be getting lousy gas mileage. It has a slight shake to it which may be a fouled plug or injector, but I haven't had a chance to check anything yet. A few months ago I replaced the spark plug wires, plugs, cap & rotor, but the truck hasn't ran quite right since I bought it. The catalytic converter had been replaced with a straight pipe (courtesy of the previous owner) and the air pump has been by-passed with a shorter serpentine belt, so it isn't being ran anymore either.

There aren't any codes showing up with a code tester, so I don't know where to look. Does anyone reading this have any ideas of things to look at, check out, adjust or replace that might smooth out the engine and improve mileage?
Thanks,
Tom