I have a 1951 Farmall Super C, new carb, but after the tractor warms up the tractor will start to loop. Meaning, the lever behind the carb starts asking for fuel, then more air, then fuel, then more air. Basically; it runs fine, then runs rough because it has to little air/ then runs fine, all within seconds. I jump off the tractor readjust the air/fuel needle valve and the tractor runs like a champ for a few minutes, but the needle valve keeps backing out to the wrong setting. THe valve will not back out totally, but enough to cause the rough running. What can I do to stop the backing out? I thought about lock tight or finger nail polish on the threads, but is there something else I should do? Thank you all.... the tractor has new plugs (gapped correctly), new wires, new rotor, new distributor cap, starter, coil, new fuel, fuel tank is cleaned out, fuel line is cleaned out, sediment bowl is clean...