Onan BFA - Spec.C, 4kw generator (gas)

Has been running super (350hrs TT) & regularly (at least 1/2 hr. every two weeks)

Starts great, runs great unloaded for about 5-7 minutes, then starts to stumble & miss a beat or two every few seconds while still unloaded. A/C start-up load will kill it in short order, but an unloaded restart works fine for a few minutes before it starts to stumble & hunt rpm again.

Good fuel, new filters, new plugs, new coil & wires.....no improvement.....

What's up with that & what's next?.....Thanks!

I had a stumbler last year after about three days running time and found the choke working itself closed for some reason. I dialed the housing back and the choke opened and she ran fine.

I am a big believer in Red Line Fuel System Cleaner. I have seen it do wondrous things on a number of generators. It will loosen up crud you didn't even know you had. Jag V12 owners particularly love it for those itty bitty injector orifices.

Also check governor (and choke, as Danny mentioned) linkages for security and freedom of movement. I like Dri Slide on all pivot points.

Is your exhaust clear? Do you have a spark arrestor that can be removed for testing? Heck, take the muffler off, too. Does it have a solenoid fuel valve?

Does it smoke when it stumbles? How is the fuel pressure? How is the float level? Does the coil have a real good ground? Have you adjusted the frequency? It should be a little above 60 with no load.

My Onan experience is a number of years back, but the most common memories are carb and governor.

The Onan in my SOB (Spec E) also had stumbling/surging issues, though not to the extreme you're suffering. At least I could start the A/c and then the microwave - but the governor would hunt.

I found the points were a mess.

A full tuneup (Points, plugs, condenser) got 'er running, but then I spent a bit of time resetting the governor and mixture, which had been cobbed to compensate for the worn-out points.

I found this was far more succinct than the Onan manual for resetting everything.

Anyway, now I can flash it up, and start the A/C and microwave after a couple of minutes warmup, and it runs smoothly.