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!