Onan 4000 generator fails after running a while

[José Snyder]

Hello I have an Onam 4000 generator I changed filters and gasoline pump, carburetor and oil, when starting at idle or with load it works perfect, but since it works for a while it begins to fail, has anyone had this problem? How have they solved it?

 

[tlmgcamp1]

While it is a completely different machine, I had a snowblower that did the same thing. It turned out to be a blocked vent in the gas cap that didn't allow the carburetor bowl to fill up fast enough to keep up with the need of the engine.

I would suggest that you look and the flow of fuel into the carburetor. If it has an automatic choke, maybe it is closing up too much as it warms up.

I am not a mechanic but simply offer a few ideas. Even a blind squirrel finds a nut not and then.

 

[Mark_K5LXP]

What revision is it? 4K's made for the last 25 years or so have built in diagnostics that will flash a code if the generator stops when not commanded to do so.

Mark B.
Albuquerque, NM

 

[DonTom]

Hello I have an Onam 4000 generator I changed filters and gasoline pump, carburetor and oil, when starting at idle or with load it works perfect, but since it works for a while it begins to fail, has anyone had this problem? How have they solved it?

If I were to guess, I would say it is going off frequency, which will shut it down. If it's less than 25 years old, it will most likely blink the codes on the light on the genny itself to tell you what the issue is.

If too old for that, I would start by checking its frequency and voltage with a kill-o-watt meter for the time it is running and see if it is going out of spec.

-Don- Picacho, AZ

 

[José Snyder]

Thank you for your kind response, it doesn't turn off, it just lowers the rpm and makes explosions. If I turn off the AC or what's charging, it fails much less. If it's out of range, how do I adjust it?

 

[Mark_K5LXP]

If it's misfiring there's no adjustment for that. Could be mixture or spark, generally with these things it's a carb issue, though over time any number of failures could occur to cause the engine to run badly. Maybe start with a compression check, no vacuum leaks, check the plug for rich/lean condition and verify static timing. The fuel level in the float bowl should be consistent, if not then there's a fuel delivery problem which could be the pump, or a failure of the line into the tank. Assume nothing, even stuff you think is OK is suspect until proven OK.

Mark B.
Albuquerque, NM

 

[DonTom]

If it's out of range, how do I adjust it?

First get a thingy such as a Kill-O-Watt meter and run an extension cord to the genny location and plug in your meter, so you can read the frequency.

Then notice the linkage on the carburetor. Two rods, one goes to the choke, the other farther back will take you to the RPM adjustment screw. Adjust for just above 60 hz if no load, such as 63 or 64 hz. The idea is to get the RPMs to be 60 cycles with a moderate load, which can be a little lower with a heavy load and a little faster with a very light load. Should be within 4 or 5 hz of 60 hz regardless of load up to the continuous 3.5 KW load spec. A little less max load if you're at a higher elevation.

However, by what you explained, your problem is more likely to be what Mark said above.

-Don- Picacho, AZ

 

[Kirk]

I changed filters and gasoline pump, carburetor and oil,

Did it run as it should before you did those things? Why did you replace the fuel pump and carburetor?

 

[CharlesinGA]

Thank you for your kind response, it doesn't turn off, it just lowers the rpm and makes explosions. If I turn off the AC or what's charging, it fails much less. If it's out of range, how do I adjust it?

Backfiring?

Is the carb OEM or a Chinese knockoff? Is the fuel pump OEM or an aftermarket pump. These generators are quite sensitive to fuel pressure, and aftermarket pumps are known to cause issues.

Charles

 

[DonTom]

These generators are quite sensitive to fu el pressure, and aftermarket pumps are known to cause issues.

Just make sure the fuel psi of the pump is very low--in the same range as the stock pump, around 3 psi.

Is the carb OEM or a Chinese knockoff?

I have the $40.00 Chinese carb and a fuel pump I got at AutoZone. All that works fine.

While I did recently (last November) have a problem at 7K feet elevation, I believe that was because of a valve with less than 0 clearance. IOW, not enough compression because the valve would not fully seat.

I did, however, do some load testing here after a valve clearance adjustment. I also changed the carb with another Chinse carb, but that was only because I broke off the plastic on the elevation adjustment making it difficult, but still possible, to adjust.

I will soon load test it again at 7K feet elevation. I will probably go to boondock in the same forest north of Flagstaff which gave my genny
trouble last time.

-Don- Picacho, AZ

 

[Mr Lars]

Hello I have an Onam 4000 generator I changed filters and gasoline pump, carburetor and oil, when starting at idle or with load it works perfect, but since it works for a while it begins to fail, has anyone had this problem? How have they solved it?

When you say ‘it runs for awhile’, what does that mean, 1 minute, 10 minutes, 1 hour? How many electrical items are you using when it starts failing?

 

[Jubilee]

Suggest a temporary piece of clear tubing between filter and carburetor to check for air bubbles or vapor.
This will check integrity of fuel line all the way to the fuel level.

A pinhole or crack can be repaired quickly with a drop or two of “Seal All” for temporary or semi permanent repair.

Vapor bubbles can usually be eliminated by insulating or rerouting ( or both) fuel line.

 

[TheBar]

Running fine for a while then failing can definitely be caused by overheating.

 

[José Snyder]

Did it run as it should before you did those things? Why did you replace the fuel pump and carburetor?

No, first there was no gasoline coming out of the pump, then it worked until I changed the carburetor, I changed oil because it was dirty, the spark plug because of the fault, last night it took longer to start the failure, I will turn it on more times before I start moving adjustments, do you think it is okay?

 

[Bobtop46]

1. Fuel pump and filter.
2. Fuel lines. (doubt as it starts and runs for awhile.
3. Generator specific problems mentioned above, Misses, backfires, frequency etc.

 

[Kirk]

I believe that you have some rubber sections on the fuel line and I have seen those collapse due to the suction of the fuel pump that is mounted on the Onan engine. I would check those or maybe just replace them with new ones. That doesn't sound like you needed to replace the carburetor so you may want to put the original one back. Doing so might eliminate the replacement as a possible cause.

 

[José Snyder]

Cuando dices 'funciona un rato', ¿qué significa eso, 1 minuto, 10 minutos, 1 hora? ¿Cuántos aparatos eléctricos está utilizando cuando empieza a fallar?

10 min +/- only microway or ac.

 

[José Snyder]

Cuando dices 'funciona un rato', ¿qué significa eso, 1 minuto, 10 minutos, 1 hora? ¿Cuántos aparatos eléctricos está utilizando cuando empieza a fallar?

Creo que tiene algunas secciones de goma en la línea de combustible y las he visto colapsar debido a la succión de la bomba de combustible que está montada en el motor Onan. Los revisaría o tal vez simplemente los reemplazaría por otros nuevos. Eso no parece que necesite reemplazar el carburador, por lo que es posible que desee volver a colocar el original. Si lo hace, podría eliminar el reemplazo como una posible causa.

I believe that you have some rubber sections on the fuel line and I have seen those collapse due to the suction of the fuel pump that is mounted on the Onan engine. I would check those or maybe just replace them with new ones. That doesn't sound like you needed to replace the carburetor so you may want to put the original one back. Doing so might eliminate the replacement as a possible cause.

Initially it did not start, there was no gasoline after the pump, change filters and pump and there was already gasoline, but it did not start, change carburetor and it started, but it presented this failure, change the spark plug and it remains the same, I will put another transparent filter between the pump and the carburetor, to monitor the gasoline, I have noticed that every time I turn it on it takes longer to fail.

 

[Kirk]

I have noticed that every time I turn it on it takes longer to fail.

Is there some reason for converting my response into Spanish? Perhaps if you start it enough times it will fix it's self?

 

[JudyJB]

What altitude are you at? This took me a few years to figure out, but mine would not reliably run at higher altitudes. Turns out that it has a tiny, almost hidden switch to adjust it. There are two higher settings, and in Colorado I switched it to highest one and it worked perfectly!! Switch is on front after taking cover off, but was behind a cord.