I have a Miller 225G plus. it idles up good and

The toroid transformer (looks like a donut) has wires, and connectors which feed a small voltage to the idle module. Since works with grinder, I'd look for corroded connections including rusty grounds. It takes one volt ac to idle mod to trigger.

it will stay running

These rigs are notorious for developing bad connections in switches and wiring, as well as getting stuck or burnt brushes in the generator. If it has been sitting for some time, I would definitely do maintenance on the brushes and check switches for rusty or burnt contacts. http://www.millerwelds.com/om/o175104b.pdf

Check your machines fuses. For location of fuses look into your owner's manual.

A welder does not speed up when an arc is struck. It is normal for a gas welder to stay at a high rpm. If it did not, it would be very hard on the machine as a high amperage is needed to start an arc for welding, and to be consistant. Unlike a generator which will "idle down" when not in use.

altenator. altenator is not working. you need a new one and probably a battery at this point.

probably the AVR card produced over-powering and then went down...try replacing it..
i hope i helped...send me an email for more details

You can get this manual from the following link http://personafile.com/Miller-Welding-Equipment-BOBCAT-225G-P9207349821373.htm

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Make sure that the idle is set at 40Hz. You do this by placing a frequncy meter on one of the 120Vac outlets. If frequency is low, the engine will run rough at idle. There is a 3/8 nut next to carberator that needs to be turned to the right to increase the idle speed.

If the idle is not working, then it is either you idle solenoid attached to the carberator or your idle board is bad. If you apply 12Vdc to the idle solenoid is should move, if not then the solenoid is shot, no good. If it works then you idle board is bad. Also, check to see if you are 60Hertz by measuring at the outlet with a frequency meter. I recommend a Fluke meter. You should have 120Vac and 60HZ at the outlet. If the frequency is low then your engine speed needs to be increased. The idle solenoid could be causing all these problems.