Bobcat 250 burns small rod just fine larger rod has problem motor dies down

google miller bobcat 250 welder generator manual

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https://www.manualslib.com/manual/1675586/Miller-Bobcat-250.html

It has been my experience the best flat welding rod is 7024. It is easy to use and forgiving of contact sticking. It has a heavy coating making it easier to keep a consistant gap between rod and workpiece. I always liked 7018 for vertical welding. I hope this helps you out.

Just had a quick look at the specs of your Bobcat welding/generator. It is quite a tool and offers both AC and DC MMA as well as other types of welding and a high capacity AC generator.. This suggests the Bobcat is far from an ordinary engine driven welding set that is typically DC only.

It is probably an AC generator coupled to inverter technology that gives it the capability of offering those different welding options. That means there is some quite fancy electronic circuitry inside.

I would guess if the generator is working well the lack of welding performance is likely to be caused by an electronic problem and is a job for a specialist.
Checking the OCV or open circuit voltage between the welding ground and electrode should provide a clue - according to the specs it should be 40 volts on both AC and DC.

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Why does my miller bobcat welder generator not stay at an idle when swithed to low idle, insted it goes back and ... Won't strike an arc,fuses ok brushes and slip.

Never try to fool a security breaker. You could damage the compressor, if something is wrong. It could over heat and burn out. Try to fine what is wrong or if you don't mind to buy a new one, kill it with a 30 Amp circuit breaker.

My welder will only run above full at full line it dies why

It might be missing a stabilizing spring on the the throttle shaft to eliminate accumulated play in the components.

Otherwise if no smell of leaking LP is noted, there may be a small vacuum leak in the induction system- can be found with carb cleaner on an idling engine- the area of such a leak when sprayed will smooth out the idle and speed it up momentarily.

If you do smell LP, the regulator likely needs repair or a hose needs replacement..

fuel blockage or filter

What model of suitcase? Could be the contactor in the suitcase, could be the mig gun. Are you using the CV setting on the Bobcat, instead of CC? Is the Bobcat on the high CV setting and the dial most of the way?

your carburetors might be dirty blocking flow to the high speed jets causing it to bog down