Miller 325 CH730 idle surge

Engine is in a 325 miller trailblazer, model CH730-3267 s/n- 4317807711.

I bought this welder with a surge at idle a slight rpm swing at full rpm. Figured it was a carb issue so ordered a rebuild kit from Kohler and tore it down not finding any kind of dirt or debris anywhere. Resealed everything and replaced jet, replaced fuel shutoff solenoid, replaced float needle. Reassembled and reinstalled. Started welder and still surging. Can pull the choke out partially and it smooths out and runs fine. So it’s in a lean condition, removed all the fuel lines and replaced, new filters, oil changed etc just to rule that out. Welded with it and it seems to be low on power due to the machine should idle back down on low amps to compensate the low load. Engine idles down but comes right back up to full speed. So I believe it’s a little low on power. Removed both plugs and they were both black probably due to running lean? Replaced them and still same issue. Went ahead and checked spark with my timing light. One cylinder flashes good and the other I can’t seem to get anything to show on my timing light. Unplugged that cylinder and don’t hear any change in the engine. Unplug the cylinder with the good spark and engine doesn’t die or lose speed at idle but definitely doesn’t sound like the cylinder is firing very strong or like the opposing cylinder. Does that have to do with a coil going bad? Is it a bad head gasket by chance causing all this? I’m lost and don’t want to keep throwing parts at it for no reason. Thanks!

I have one plug that is black and the other that is clean. I unplug the plug wire on the plug that’s black and I can push the choke in and other than the misfire it will run fine and the exhaust doesn’t sound as choppy sounding. Hook the plug back up and instantly starts acting like it’s sputtering and have to pull the choke out about half way again. Is this leaning towards a head gasket or head issue?

Did compression test and both cylinders were at 160 psi. Did cylinder leak down and both cylinders held steady 100 psi and in the green for over 5 minutes. Changed both coils and still having issues. I’ve rebuilt the carb twice now with Kohler parts and still not showing any change there, swapped the fuel shut off solenoid on the carb and no change. Checked both push tubes and they aren’t bent or anything. The only issue I have seen is one cylinder around the exhaust valve has what looks like burnt oil around the valve spring and has ran down the oil drain hole. Dark brown and the rest of the head and other head is spotless. Can that be my issue? It doesn’t smoke or burn any oil is the other thing that throws me off. I don’t know what else to check?

I think you have one problem as it appear to leaking exhaust gases back into the engine via a bad valve guide. Just a thought as actual checks are in order.

Any help here? I ordered one head for the one with carbon up top but need advice as weather to do both?