"First, my apologies for posting this issue here. Couldn't find another related forum.
I have a 1988 Onan 6.5 H-series Marine Gas Generator. Low hours. 2-cylinder opposed with aviation plugs and braided/shielded ignition wires from coil. Engine starts and runs well at idle with little or no governor hunting; also runs well with full load, including 2 AC systems, refrigerator, etc., that push it to 20 amps. Carb clean; points clean and gapped properly; fuel lines clean; new fuel-water separator; idle and heavy load mixture set to factory defaults; engine temp good; closed-system coolant clean; sea water pump working well.
Problem: When stepping up from idle to 15 amps load, Genset works great. When adding load greater than 15 amps, genset backfires, coughs, governor hunts erratically, but eventually achieves desired load and holds steady voltage/amperage.
Then, still at full load, genset will eventually die as if it is out of gas (tank full).
Any help is appreciated. Need to resolve backfiring, governor issues, and reasons for engine quitting under load. I have ordered new plugs, but the old ones are clean and gapped properly. Trying to find new choke assembly, but it starts fine and idles smoothly, so that doesn't seem essential to these issues."