I recommend you clean the carb and I'll tell you how and why.
You may run into many recommendations to replace it, but the original carb costs several hundred dollars and even the eBay knock-offs aren't cheap. Besides, the new carb governor setting must be readjusted to run the armature at exactly 3600 rpm to keep the precise 60 cycles per second volts a.c. rate for reasonable efficiency and swapping carbs is actually far more complicated than simply cleaning it.
The main reason folks think the carb needs replacing is the factory recommends it,
but there is a catch why this is so. The factory must strive to only sell parts (or procedure!) if they must meet EPA standards (Onan does) and there is a good chance that someone will misadjust the fuel ratio during cleaning. The fuel ratio limit altitude range is locked by the plastic cap around the main jet and pre-set by the factory. This cap must come off for adequate access to cleaning. Try not to break it, although it will run fine without it, sans legal EPA appearance
. It's pressed on and usually can be carefully pried loose.
The carb doesn't really have a separate small idle jet because it always runs at 3600 rpm for 60hz. Offhand the chokes seem to stick off and start hard on these gens from dragging on the housing. I bent my rod link and it works better but could use a little more to be perfect.
For disassembly and cleaning, follow the video below:
The only YouTube video that knew what it was talking about on a QV4000 was:
by Jeff Berry. Take care on the 'puter audio volume because he talks soft at first, then at 40 seconds, starts his not-so-quiet Onan Microquiet 4000, so the viewer can hear what a LOUD lean fuel surge sounds like.
I was able to easily get my carb off just as Jeff says, although it's a bit tedious to keep carefully twisting the 8" of solenoid wire lead along with the unscrewing solenoid, so it can't break off. Pay close attention to the washer positions on disassembly. There are two washers, one inside the bowl and the other outside. Take care with loosening the bowl gasket as I believe there no new ones to be had. Where Jeff disassembled what he calls the jet (I think it's an emulsion tube and/or solenoid valve seat) from the main solenoid body, I did not disassemble since there isn't anything to clean in it, so I didn't bother.
Where Jeff recommended backing the "altitude adjustment screw" out as far as he could to the plastic cap stop, I regard this "screw" as the adjustable mainjet needle restriction and, after marking the cap's position with the brass
, did carefully pry the press-fit altitude cap slightly and evenly away from a tight fit against the bowl housing using a sharp jack-knife, so that I could just barely insert a pair of small screwdrivers in to finish removing it square. This plastic cap pops loose at about only a 1/8" gap from seated. There is a milled gear-like spline to hold it on the brass when jammed back in place.
Once this is done, the brass needle screw simply turns all the way out and you will have excellent access to spray the jet orifice out with aerosol brake cleaner. You could use carb cleaner, but it will eat some plastics and paint when it drips, so be careful. You likely won't need more than brake cleaner anyway. WD40 would probably even work.
Like many jet needles, this one works at about 1 1/2 turns out from bottom, but if you marked the original position per the cap, you can set it back to exact factory EPA range settings without the cap
as if the factory-set plastic altitude cap is still in place. The altitude cap is just a pair of rotary stop positions to limit the main brass needle adjustment.
My jet orifice was noticeably crusty and sprayed clean with a slight effort. Thereafter I re-assembled the clean carb without the plastic EPA/non-tamper altitude cap and adjusted it as though the cap still existed for clean EPA running and fuel efficiency. My non-tamper
cap now hangs on a small wire on the gasline entry connection to be instantly re-attached (pressed back on) should the need ever arise for EPA compliance in some states. In this manner, the next cleaning will be simple, since I can simply take the entire bare brass needle out and just quickly spray the orifice clean without disassembling the carb at all, even in the boondocks. It's easy to get at.
So why would I need to do that? Because it can easily happen again if the QV4000 gets so hot it boils the fuel after shutdown. Gasoline boils at 100 to 400 degrees F, depending on which gasoline molecules are present. After 400, I presume it's not automotive gasoline anymore, but a heavier fuel.
I attribute my own problem to shutting down without a sufficiently long no-load cooling run. The previous last use had been in our driveway in higher than 100į F (107?) temps for A/C. When done, I only let it run unloaded for about 3 or 4 minutes to cool down. Far less than 30 days later, I started it again and noticed it was already clogged up and surging.
Because of the extremely hot day, I believe the high residual internal temp rose so high when the fan quit, that the massive hot internal metal boiled all the fuel out of the carb within minutes, leaving a crusty residue behind like a 2 year storage dry-out. I haven't checked shut-off heat rise with a thermometer yet, but from experience fully opening the housing to facilitate cooling right after shut-down, I'd guess I might be able to nearly bake a pizza in there by the excessive residual heat after shutdown, since it only fan-cools when running (draws all air right across the carb!). The more amazing part is that it ever works twice in a row in barely warm weather. I guess the cool down is critical. Fuel injection would flat out cure this. For the money, I don't know why it isn't on there... unless Onan is getting rich selling new carbs.
The reason engines surge during lean fuel is because fuel passages suffer from momentary inertia when the throttle is cut (like pipe hammer) and also as soon as the air flow is reduced otherwise, such as from starvation dying. The inertia-spurted mix becomes temporarily rich, so since the governor has the throttle wide open to speed up but suddenly shuts it from over-speed, it fires, dies and periodically surges beyond correct speed. It is this inertia/governor cycle that causes the on & off throttle effect. I do not believe it is ever from being too rich. Replacing a sinking float valve would reveal that the carb jet is also clogged from long term wet storage, when slow evaporation leaves residue.
All the other videos usually just recommended running sea Foam and such through, which didn't help my clogged 'leaness 'surging. For what is mostly an engineering problem error, most people blame ethanol added to the fuel for gum-up (including Onan). Onan actually recommends perpetually treating the entire 55 gallon RV tank with their (or a) expensive fuel preservative, or running non-ethanol fuel, both of which are preposterous.
FWIW, the ethanol clog problem has been fake news from the beginning. Consumers must expect a negative reaction from any method that attempts to "steal" a 10% petroleum market share. Alcohol is what we add when we get a water problem in our gas, not what causes it. The water absorption of alcohol is what frees an iced up gas-line and is simply called de-icer, Heet
brand for example. The absorbed water is then blended with normal fuel and passes harmlessly through the system. An exception to harmless alcohol is continuous methanol use, but not ethanol.
Best to keep fuel tanks full when stored, to deter condensation and subsequent rust in the tank. Usually the first thing to rust is the fuel gauge sender mechanism.