I just bought a Miller Bobcat 225G Plus (1996 model or so). It starts and runs, but continuously idles up and down, up and down. It will also not run at weld speed when I turn the switch to "rabbit". Idles up and down regardless of switch position.

I have done a complete tune up (plugs, wires, new gas, oil & filter, air filter, fuel filter, etc) and I have also replaced the throttle solenoid and the problem persists. It will even idle up and down (and the thottle linkage moves back and forth) with the throttle solenoid completely detached.

Any suggestions on what to try next? Thanks.