Onan 4000 Generator won't start

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FunNSun

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Jun 27, 2022
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Kansas
My Onan 4000 generator won't start. I have installed a new carb, a new spark plug, cleaned the battery terminals, made sure the house batteries are fully charged. The generator tries to start but floods (I don't have to prime the generator). It gave an error code (that I forgot) but I no longer get an error code. I had the generator in the shop, it worked fine when we picked up the RV but a few months later, when the weather got warmed, the generator is back to flooding and won't start. Any ideas?
 

Ray-IN

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That sounds expensive already, with no solution. Have you visited a Cummins/Onan repair center? I would think the shop that worked on it has a limited warranty on their work.
 

Isaac-1

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By chance could someone have installed the wrong fuel pump on it, Onan carburetors are very sensitive to having the correct (very low) fuel pressure.
 

Ex-Calif

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Sounds like the mixture is way rich. Or as Mark said the auto choke is not disengaging. If it floods before lighting off it is more likely to be mixture.
 

Matt_C

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SE - Mich
Kansas,
How Old is the generator? Mine is a 4.0 Onan that is 50yo and started reliably until yesterday. (I will fix it today.)
Is this what it was doing before you took it to the shop?
If it is flooding at start up. As Ex-Cal said, make sure that the choke is opening.
If it appears to be, then try holding it open when you crank from the switch on the unit.
If it still floods, then it is probably the carburetor needle and seat. This cam be repaired most times or it might just be a POS in the seat.
Matt
 

Isaac-1

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One more thing here, did you install a genuine Onan carburetor which typically sells for around $300, or did you install one of those $35 Chinese knock offs, with highly variable quality control?
 

FunNSun

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Yes, will check that. More to come. Thanks
I checked the choke and adjusted it. If I hold the choke open, the generator runs but it revs up and down. When it idles, the generator dies. It's not flooding at least. The generator gave a code - needs serviced. I have installed Cummin/Onan carb. The generator is a 2019, it has 6 hours on it. I put 11 additional hours on it trying to fix it.
 

Ex-Calif

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I checked the choke and adjusted it. If I hold the choke open, the generator runs but it revs up and down. When it idles, the generator dies. It's not flooding at least. The generator gave a code - needs serviced. I have installed Cummin/Onan carb. The generator is a 2019, it has 6 hours on it. I put 11 additional hours on it trying to fix it.

Are you holding the choke open or the butterfly. Sounds like you are opening the butterfly, the generator is hitting the rev limiter and hunting. The fact it does this is good but dying at idle - when you release the butterfly still indicates a carb problem.

Have you located the mixture settings and adjusted them per the book? I don't know the Onan (someone will chime in) nut most small engine carbs suggest screwing the mixture all the way in and then backing out 2 turns for the initial start up.
 

FunNSun

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Jun 27, 2022
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Kansas
Where do I find the "nut" mentioned above? I have looked and don't see it unless it is the nut on the float bowl.
 
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