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Old 10-24-2020, 08:54 PM   #1
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Onan QV4000 starting only by choke but then surging under load

Our genn now wonít start without manually choking it , just tapping/quickly closing the choke lever, cold or hot. When it starts it idles just fine. However when we add a load, AC or microwave, it surges a lot. This generator always started easily for us. I would start it occasionally and run a load once a month.
I thought for sure I put stabilizer in the main tank. Maybe not? We plan to desert camp in a week and Iím wondering besides the annoying surging and having to go outside to close the choke each time to start is, until I can get in line at the shop are there any issues to the electronics by running it itís way?
My attempt to repair was to spray carb cleaner where I could, run a mix of Seafoam an fuel for 30 mins, let sit etc.,per others recommendation, and pull the bowl to check the floats and spray more.

I donít want to buy a portable genn but this trip must happen.

I hope I hear good news from you??
Thanks.
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Old 10-25-2020, 08:29 AM   #2
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Welcome to the forum its a great place to get your questions answered. What rig do you have? When my generator Onan 5K was surging I added seafoam to the main tank in the ratio recommended plus a little and it took about six hours of continuous running to clean things up and run smooth. that six hours was with the generator loaded.
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Old 10-25-2020, 08:40 AM   #3
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The carb is probably partially plugged up internally. No amount of carb spray can fix it without carb disassembly. In the meantime, if the carb has an accessible mixture adjustment screw, you might try adjusting it richer to get the engine to run right.

I know Onan says for short term storage to run the engine periodically. I donít do that (partly because the RV is stored inside a building), but what I do is make sure to run the carb out of fuel anytime the genny wonít be used for a few weeks or more. Installing a fuel shutoff valve right at the generator makes this easy. Another way you could do it is to install a switch to kill the generatorís fuel pump power, so it will run the carb out of fuel before storage.
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Old 10-25-2020, 12:29 PM   #4
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Thanks to both for the info so far. We have a 2015 30ds. The genn is a Cummings Onan RV QG4000. I ran the SeaFoam mix for 30 mins heavy on the SeaFoam, let it soak for 30 and ran another 10, from a separate 2 gal can direct connect to the genn. This carb has an altitude adjustment. I learned about the bowl drain screw, the hard way.
My concern is if I run it like this is there other possible damage to the genn motor or connected electronics?

Thanks
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Old 10-25-2020, 12:59 PM   #5
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If the generator is surging, it is likely running rich. I bought a used 2007 RV in 2016 with a Generac 3600 watt generator that had hardly ever been run (because it didn't generate anything due to a bad voltage regulator). Anyway, it surged when running, and I found that the plastic float was half full of absorbed gas, causing a high float bowl level. The price of a new float was half the price of a new carburetor (from Amazon), so I just replaced the whole carb and it has run fine ever since.
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Old 10-25-2020, 02:31 PM   #6
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New carb.

On eBay a new carb is only $36, held on with 3 or 3 nuts, 15 minute change out.
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Old 10-25-2020, 05:49 PM   #7
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One other possible cause not yet mentioned is the fuel filter.
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Old 10-25-2020, 06:08 PM   #8
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When it is surging, can you tweak the choke on slightly? If it improves, it was too lean (fuel filter or gummed up fuel jet). If worse, it was too rich (flooded float or float valve leak).
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Old 10-26-2020, 06:24 AM   #9
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SoCalRV4fun,

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:
https://youtu.be/Ph_C6XF2TCU 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.

TRIVA:
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.

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Old 10-26-2020, 11:35 AM   #10
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Robie Crossing my fingers but I ordered a $60 carb on Amazon. I know I might be asking for trouble but now Iím out of options. It has good reviews on the web. I saw 2 fairly good videos on YouTube from rjeepster showing how to replace it.
TGR I took off the fuel hose and while priming it seemed to be pumping well.
Underdog I will try to tweak the choke first before taking the plunge to replace it.

Wes-I have removed the bowl and sprayed as much as possible. I should try the altitude screw since I have to wait for the replacement carb tomorrow 9/27 two days before I leave and boondock for 3 days some of it in the above 90degree heat.

Thanks again all. This is another $ lesson learned to drain and prep the carb. At the same time Iím dealing with an ATV quad with gummed up carbs for the same doh! reason.
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Old 10-26-2020, 10:48 PM   #11
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Quote:
Originally Posted by SoCalRV4fun View Post
Robie Crossing my fingers but I ordered a $60 carb on Amazon. I know I might be asking for trouble but now I’m out of options. It has good reviews on the web. I saw 2 fairly good videos on YouTube from rjeepster showing how to replace it.
TGR I took off the fuel hose and while priming it seemed to be pumping well.
Underdog I will try to tweak the choke first before taking the plunge to replace it.

Wes-I have removed the bowl and sprayed as much as possible. I should try the altitude screw since I have to wait for the replacement carb tomorrow 9/27 two days before I leave and boondock for 3 days some of it in the above 90degree heat.

Thanks again all. This is another $ lesson learned to drain and prep the carb. At the same time I’m dealing with an ATV quad with gummed up carbs for the same doh! reason.
SoCalRV4fun,

I hope the new aftermarket carb works good for you. A lot of folks have had good luck with them. A couple I tried on yard machines didn't seem to have exactly the right mix for full power and the throttle settings ran at slightly different speeds until I fiddled with the governor. I think my snow-blower initially ran at closer to 4k+ rpm for instance, but still bogs down easier now. Since these carbs have an idle circuit, they are a little more complicated than the Onan. The snow-blower governor also surged, but it was because the small idle circuit plugged up.

Meanwhile, all my QV4000 residue was exactly in the orifice where the main needle seats. Jeff recommended backing this altitude adjustment slightly out to it's stop to clean it, but I can't see that working well at all. Maybe it will... he said so. But when the needle is entirely out, it's obvious as heck what the problem is and much easier to clean. Remember, if you break the plastic altitude stop piece, it will still run like new when clean. You just won't have the OEM EPA situation or look if it matters. The new non-OEM carb is not necessarily certified either so I wouldn't worry about either, at least where I live. In my experience most carb clog failures are similar. They exist in one critical metered area of the carb, usually towards the bottom. Find and open that area and you've got it.

I mentioned that idle circuits (like in your ATV) are more of a problem. In about the early 1980's, carb idle circuits began to be further restricted in size. Over the years, this trend started on the street and eventually affected all off-road stuff. This reduced size causes the idle circuits to be exceptionally vulnerable to clogging. When an idle circuit is used, such as on older autos, many yard machines and ATV/motorcycles, the engine only receives the correct high speed mix when it gets fuel from both the main jet and the idle jet at once. When the idle jet clogs up, the engine may start and run rough on the larger sized main, but it won't have full power and may bog easily because it will be too lean.

Fixing these carbs may be a little harder depending on where the clog is. Having them soaked in cleaner doesn't always work on hidden passages. Furthermore, the small idle jets are so tiny, ordinary redneck "cleaning-wires" (bread-wrapper ties etc.) may not fit into them during mechanical poking. If you run into this, the best small wire can be found in ordinary flexible stranded lamp cord. These hair-like wires are usually small enough to fit the idle jets and passages found in ATVs for example. Really miniature weed-whackers, not so much.

Wes
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Old 10-27-2020, 04:24 PM   #12
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Again thanks to all for the input and advice. I got the new replacement carb(I was please to see the box looked professional instead of a plain brown cardboard box)and Iím heading over to the RV to do some troubleshooting and worst case replace the carb. Question this new carb came with an attitude adjustment setting range from 0 to 5000 feet to 10,000 feet. My original car has altitude adjustment from 0 to 3000 feet to 6000 feet is this a big difference. Also in this next photo what is this extra part for? My assumption is it replaces or bypasses the electrified part on the bottom? Click image for larger version

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Old 10-28-2020, 06:53 AM   #13
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Originally Posted by SoCalRV4fun View Post
Again thanks to all for the input and advice. I got the new replacement carb(I was please to see the box looked professional instead of a plain brown cardboard box)and I’m heading over to the RV to do some troubleshooting and worst case replace the carb. Question this new carb came with an attitude adjustment setting range from 0 to 5000 feet to 10,000 feet. My original car has altitude adjustment from 0 to 3000 feet to 6000 feet is this a big difference. Also in this next photo what is this extra part for? My assumption is it replaces or bypasses the electrified part on the bottom?
Thanks for the pictures. The carb is a very decent looking product. I'm impressed they sent the solenoid with it. I think you're correct, the extra brass is probably a non-solenoid option.

The solenoid only does a few things. It looks like you could probably leave it off if you like. I don't think it serves as a safety shut-off, such as for low oil for instance.

The solenoid positively shuts the gas off when the ignition is cut, even though the heavy armature (flywheel effect) keeps it pumping for a bit. If these units are ever installed as a stationary(?) generator with gravity feed fuel, the solenoid shuts off fuel that would drain the tank dry if the float sticks open. If the engine is hot, the solenoid insures that it can't diesel on. It seals the fuel tap from the atmosphere so tank fumes can't leak directly out. Vented fumes are collected by the carbon canister under the RV hood and burned upon start. I forget what else.

The altitude range adjustment shouldn't make any difference IMO. Adjusting my range didn't seem to make a noticeable difference on my QV4000, but it wasn't properly loaded when I did it either. Normally in the past, with yard machines, richer gives a noticeably stronger exhaust note but also affects fuel consumption to a fair degree.

I liked old Tecumseh engines with an adjustable float-bowl carb for power and economy, but old Briggs engines with pulse carbs were more trouble-free. The Briggs used almost twice as much fuel. It wasn't the money, but the nuisance of refilling the tank halfway through mowing.

Wes
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Old 10-29-2020, 12:31 AM   #14
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Latest update(hopefully the final). I installed the replacement carb today. WHEW! It was challenging. Special help was the Rjeepster utube video. It started right up and works pretty well. It surges very slightly but runs all of the required appliances. It also starts without having to choke it. I read that it may need to be fine tuned at the governor to correct this. Iím hoping it will get me through this weekend. The only bad thing I see with this cheapie carb is the bowl drain screw was seated real tight, assuming it leaks if it was not. This would be a concern if I plan to drain it regularly. I was able to break it loose and it sealed ok.

Thanks again to all for the info and encouragement to work on this. Iím hoping to say it saved an off-road weekend designed for the kiddies.
I have other questions about this FR3 but will post later.
Happy camping!
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