I had no issues with my Onan 4k until after it sitting for 8 months or so. The original issue was the carb from fuel going bad so I took it out and cleaned it with no success. Bought a new carb and the generator fired right up and ran great. However, after about 5 minutes or so it started hunting rpm and quits running. Looking at the plug it is soaked in fuel, engine is flooding out. Went through the carb and all seems to be functioning correctly. If I throw a new plug in it, it will run fine for a few minutes and then flood out again. Also, when it starts hunting I can pinch the fuel line just a bit and it clears up and runs good again. It's definitely over fueling... I have had repair people tell me several different things and no two repair people say the same thing. I believe the generator itself is fine, but maybe an electrical problem with the computer. Has anyone else ran into this and if so, please let me know what your fix was. It's not running hot, the oil level is good, new carb and plug, air filter is new, fuel filter is new. I am running out of ideas here...
Onan Generator Over Fueling Issue
- Thread starter CWI-Guy
- Start date
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Gary RV_Wizard
Site Team
A spark plug wet with fuel says combustion is not occurring when & as it should. That means one of three things is happening:
- Air/fuel mix is off
- Ignition timing is off
- Spark is intermittent
NY_Dutch
Well-known member
Did you buy on Onan carb or a Chinese knockoff. In either event, it could be a float valve sticking open.
DonTom
Well-known member
Gary RV_Wizard & NY_Dutch,
I was thinking computer only because the carb solenoid may not be kicking off and on when needed. Or the timing of the spark is not right. Unfortunately, the only mixture adjustment on the new carb is the elevation dial. The choke is closed at start up and opens fairly quickly, so I don't think it is choking itself out either. I can try the old carb again. I never changed the mixture from when it ran perfect to when it is not staying lit for more than little while. It ran great and then it did not after sitting. I figured a little cleaning and it would be fine again. That works for everything else with carb. I bought an amazon carb that was a "replacement" for Onan carb. They both are getting the same result when I swap them out. I did blow through the fuel in tube and lifted the float and they both seem to stop when the needle seats with very little pressure applied. The new one I ordered came with a solid plug option for the bowl where the Onan has the fuel cut off solenoid (came with a new solenoid as well). Could I put the plug in or does it need the solenoid to function properly? I did test it to the battery and it is clicking in and out when power is put to it.
DonTom,
As far as the plug goes, I honestly don't remember the brand I put in. Picked it up from O'Rielly's and threw one in. It does solve the issue, but only momentarily. I did read your thread though and it is definitely food for though.
I plan on an oil change this weekend along with another new plug. I'll grab a couple different brands to eliminate the plug as the culprit. I can try the old carb again and see if the new carb is acting any different. It didn't make a difference last time, but I'll try again. I can't bring myself to spend an absurd amount on a carb and that not be the problem. If the problem can be certainly identified as the carb, then I will buy one. But until then I will hold on to the 500$ and see if I can find the problem first...
I will keep you informed, thanks for he input!!!
I was thinking computer only because the carb solenoid may not be kicking off and on when needed. Or the timing of the spark is not right. Unfortunately, the only mixture adjustment on the new carb is the elevation dial. The choke is closed at start up and opens fairly quickly, so I don't think it is choking itself out either. I can try the old carb again. I never changed the mixture from when it ran perfect to when it is not staying lit for more than little while. It ran great and then it did not after sitting. I figured a little cleaning and it would be fine again. That works for everything else with carb. I bought an amazon carb that was a "replacement" for Onan carb. They both are getting the same result when I swap them out. I did blow through the fuel in tube and lifted the float and they both seem to stop when the needle seats with very little pressure applied. The new one I ordered came with a solid plug option for the bowl where the Onan has the fuel cut off solenoid (came with a new solenoid as well). Could I put the plug in or does it need the solenoid to function properly? I did test it to the battery and it is clicking in and out when power is put to it.
DonTom,
As far as the plug goes, I honestly don't remember the brand I put in. Picked it up from O'Rielly's and threw one in. It does solve the issue, but only momentarily. I did read your thread though and it is definitely food for though.
I plan on an oil change this weekend along with another new plug. I'll grab a couple different brands to eliminate the plug as the culprit. I can try the old carb again and see if the new carb is acting any different. It didn't make a difference last time, but I'll try again. I can't bring myself to spend an absurd amount on a carb and that not be the problem. If the problem can be certainly identified as the carb, then I will buy one. But until then I will hold on to the 500$ and see if I can find the problem first...
I will keep you informed, thanks for he input!!!
John From Detroit
Well-known member
As I recall the ONAN fuel pump is a LOW PRESSURE pump... Too much pressure will force fuel into the carb and flood the engine.
Also if the float valve is not set properly.. Same effect.
I had a Van that loved to flood out.. Turns out the fuel pump was flaking and blocking the float valve open.> Finally replaced pump.
Also if the float valve is not set properly.. Same effect.
I had a Van that loved to flood out.. Turns out the fuel pump was flaking and blocking the float valve open.> Finally replaced pump.
I may try a fuel pressure regulator. Just a cheap one to see if that helps with the issue. If so, may need to replace the fuel pump. I talked to Cummins and they said that a bad pump can over pressure or under pressure. When I pinch the line off and restrict the flow, it does clear up before it starts to starve. Shouldn't have to adjust the float from factory, but maybe this one needs it. What stumps me is that I have the same issue with the old carb and the new carb, leading me to believe it is something else.
udidwht
Well-known member
Excessive fuel pressure will push the needle valve open flooding the generator. 2-3psi is all that is needed. Also never assume that a carb is set from factory. They rarely ever are perfect out of the box.
Sorry for the delayed response. The new carb only has the elevation adjustment. That's what I meant from factory adjusted. I cannot change the mixture, I should have been more clear. To be honest, I have given up for a minute before it frustrated me too much...lol. I have had three different "generator mechanics" look at it with no resolution.
To your point though, will a bad fuel pump possibly send too much pressure? If so, I will by a cheap replacement. If that works I will buy the name brand one. I'm just getting tired of buying parts and installing them only to be in the same position...
udidwht
Well-known member
Onan Generators
Restoring, operating, and maintaining vintage Onan generators. When asking questions about your generator, give your model number AND serial number. Smokstak Subscriptions, Support and Information
www.smokstak.com
DonTom
Well-known member
That will change the mixture. To make more lean, set it to a higher elevation and vice versa.
-Don- Reno, NV
I have it set to lowest elevation now because I am in Florida and just a touch above sea level. This is also where the old one was set. I will try to set it to the other extreme and see if there is a difference. Thank you!
Attachments
Just threw a new NGK plug in it, set the elevation to the highest setting. Ran for about 4 minutes, started hunting and shut down. Same ol song and dance. Pulled the plug and it was full of fuel. New filter, fuel line, and carb. Drained carb and pulled bowl, squeaky clean. Reassembled and won't start at all again. I'm sure if I throw a new plug it will run for 5 minutes again. It has to be the pump or the brain. The only things that haven't been replaced yet...
DonTom
Well-known member
jrclen
Well-known member
I had the same problem with an old motorcycle. The problem was dirt in the gas tank getting into the float bowl and not allowing the needle valve to seat, flooding the engine. I solved it by cleaning the needle valve and installing a fuel filter in the gas line.
I originally thought something along these lines. Worked on lots of small engines and usually that is the problem. Pulled the fuel line and used a temp tank to flush the system. New fuel filter, new fuel lines, and new carb. Just pulled the bowl again today to verify and is as pretty as when I installed it. My biggest conundrum is that the new carb and the old carb are giving me the same results. Kinda why I'm leaning towards not the carb or delivery system. Unless it is the pump, have not replaced it yet. May order one here in the next couple of days. I've just never heard of a bad pump overpressurizing before. Who knows, maybe it is.
Gary RV_Wizard
Site Team
I don't think there is any "brain" in an Onan 4k that manages fuel mix or flow. Not like a car engine that operates in a "closed loop", where electronics get sensor feedback and continually adjust fuel & air quantities. It's a carb, not fuel injection.
DonTom
Well-known member
Can you check the fuel PSI, could it be too high? Stock fuel pump in there or not?
Fuel PSI has to be very low for an Onan. Around 5 psi or so. Has nothing to do with the control board. It doesn't control fuel PSI, just alarms and other such stuff as forcing shutdowns because of alarms.
Are you getting any error codes?
The extra fuel on the plug could be coming in just after the engine dies for another unrelated reason. The control board can shut down the genny for many reasons, and many of them can sound like an engine problem. Such as going off frequency and back within spec and out again. The freq must stay close to 3,600 RPMs or the control board will shut down the engine. But such things will show as an alarm code.
About what year is the genny? Older ones may not have alarm codes.
-Don- Reno, NV
