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Usually the only reason a generator engine hunts under load (or any other time) is because it is running lean; in this Onan 4000 case the carb is clogged with dried fuel residue. Whatever you do, don't just start adjusting things. Adjustments seldom change and if you leave them alone, they will still be correct as soon the carb is cleaned. If the $3-400 carb is replaced, well then every lousy adjustment surely needs to be rechecked anyway, not only incredibly expensive, but an unnecessary labor nuisance as well. See the end for simply cleaning these units.
There is a long story behind why this specifically occurs and it is not because of ethanol, or anything like that. If these small engines (generators, yard equipment etc) were designed a little differently, they would run as reliably as our car and truck engines. The fuel is fine. This seems to be a point even Onan missed by some remarks I've seen in their literature about recommending exotic fuel treatments to cure a simple chronic carb problem.
Since these engines don't normally idle (they run at 3600 rpm, I think), they don't have but one jet that handles medium to full throttle to maintain precise rpm under varying loads.
That jet is controlled by a main needle that has an obvious black knob normally marked for altitude. Since almost all engines are now designed to run lean and clean, the altitude knob
limited adjustment is needed to pass all EPA and allow the gen engine to be set to altitudes from sea level to over 10k feet. Not like our now common fuel injected stuff which is automatic.
To clean this carb does not require removing the entire carb; it requires that one remove the float bowl. To do that, first unplug and unscrew the ground of the anti-leak solenoid wire that leads to the bowl bottom so you don't twist it off. (FYI, this wire is there to shut the fuel off in case of a leaky float needle. Our application doesn't need this feature since we
only pump the fuel up to the gen. Other remote Onan applications may use a gravity fuel feed, so everybody gets this useless (to us) feature.) When the wire is disconnected, remove the brass hex nut from the bottom of the bowl along with the solenoid assembly, taking care not to twist the darn expensive solenoid wire.
Don't lose any gasket washers. There are three.After one removes the float bowl, they can blow the jet out with an ordinary cheap spray can of carb cleaner that you can buy at auto stores (Walmart etc). You have to spray backwards from inside the bowl, against the needle tip hole. Watch your eyes for splash since the bowl and holes will direct spray back at you. Keep it off paint too. You may be able to unscrew the needle entirely from the carb by prying the knob adjustment stop off, although I've heard of folks breaking the tamper-resistant plastic knob, so take care. If you forget where the needle was set, it will start when 1 1/2 turns out, but better to get it exact.
Noting the
exact position of the plastic part, I removed the plastic EPA stop and unscrewed the needle entirely which allows much better cleaning access. In the future I should be able to remove the needle this way without disassembling the bowl which will save considerable time because of that confounded solenoid wire.
My generator seemed to have clogged in my driveway up right after 20 days of non-use even though I've gone several months in the past without "exercising it" (to keep fresh fuel in the bowl). Fresh fuel keeps the dried-up residue from forming. My guess is that mine
actually clogged up the next day (or hour) after I used it because I didn't realize how sensitive these generators obviously are to heat soak. By heat soak, I mean that the carb is trapped inside the shroud which gets like an oven inside when no new cool air enters to flow past it. There is a lot of hot iron in these things after they've run awhile. Could probably cook a pizza.
Onan casually recommends running their generators a few minutes after use
without load to cool them down. IOW, shut the AC off and run over 10 minutes before engine shutdown. I did do about 5, but that was apparently not long enough. It was a well over 103°F day which was the reason I ran the AC. What I should have done after AC-off is run it longer with the gen access door closed normally, then opened the door and shut the gen down from outside the camper. AND I should have left the gen door wide open for 45 minutes while the stationary gen finished cooling off.
By not doing that, I'm suspicious the carb boiled all the fuel out in just a few minutes since it was like leaving it in a closed hot oven right after baked cookies have come out. Gasoline boils at as low a temperature as 100°F, depending on winter-summer formulation. The carb could just as well have sat for a year drying out... and here I accelerated the detrimental process down to a few minutes.
My guess is that is a large part of the problem that many Onan plagued owners suffer from... a bad last shut-down. Someday I plan to actually check the cool-down curve with a pointed meat thermometer stuck through the bristle seal.
Wes
Days spent camping are not subtracted from one's total.
- 2019 Leprechaun 311FS Class C
- Linda, Wes and Quincy the Standard Brown Poodle
