Cummins Onan Generator running ruff at start up

My 4000 had similar problems. It also would not run at full load, i.e. running the microwave and AC at the same time. If I started one or the other the gen would slowly bog down and die within 20-30 seconds.

After having a Cummins Onan tech look at it, it turned out it was running too rich. He suspected it had sat for awhile and accumulated some debris around the needle valve and the previous owner kept opening the valve to compensate. Once the debris cleared out full flow was restored and it ran way too rich. It took several adjustments over a month or two, but it is running fine now.

To adjust the needle valve, you have to remove the "altitude adjustment" nob (mine is a black plastic ring set over a brass thumb screw.) Pry the ring off carefully using two flat-head screw drivers on either side. Then turn to the right to reduce the air/fuel mixture. I think mine ended up being almost one-and-a-half turns by the time it started running right.

Always make sure to run it about an hour under at least half load once a month. I've forgotten and not done it for almost two months and been fine.

Hope this helps!

Another possibility (mainly for AK49er) is insufficient priming to get the air out of the fuel line from the gas tank to the carb. On many/most Onan generators, pressing and holding the "stop" side of the switch before starting the generator will run the fuel pump to prime the fuel system. Doing that for awhile (15-30 seconds, perhaps) before starting often will help initial poor running. This is especially true if the poor running is worse if the generator has sat for a relatively long period of time (i.e. one week vs. one day, for example).

The choke linkage/system could also very well be the problem, of course, probably among a few other possibilities.

I have the same problem with mine. I just took the choke pull off from the carb and sure enough it will no hold vacuum.

ksg5000 wrote:
Have you checked the spark plugs? If I recall code 37 is one of those generic faults which can mean lots of things -- I would rule out the basics first.

Actually yes, that was the first thing I did.

nuggetthunter wrote:
Yes, the vacuum breaker is the same part. The vacuum breaker is designed to gradually open the choke when the generator is running until the unit is warm enough for the mechanical choke pull off to overcome it. You may not be able to remove the vacuum hose to check it with the breaker installed. Very short hose in close quarters.

If you need it the replacement part number is:
Vacuum Breaker Kit A043B658 (Old Kit # 170541-1222)

Hope this helps you out.

Great info. thanks!

Have you checked the spark plugs? If I recall code 37 is one of those generic faults which can mean lots of things -- I would rule out the basics first.

Yes, the vacuum breaker is the same part. The vacuum breaker is designed to gradually open the choke when the generator is running until the unit is warm enough for the mechanical choke pull off to overcome it. You may not be able to remove the vacuum hose to check it with the breaker installed. Very short hose in close quarters.

If you need it the replacement part number is:
Vacuum Breaker Kit A043B658 (Old Kit # 170541-1222)

Hope this helps you out.

Thanks for the replies. So I take it that the "choke pull off" we're talking about is the same as the vacuum breaker assembly that's located on the side of the carb that controls the choke, correct?

If you don't want to remove the choke pull off to test it, just put a piece of tubing on it and suck to see if it responds.

I have the exact unit in my toy hauler that was doing the same thing. You are on the right path with the choke pull off. Check to see if it will hold a vacuum. May have to remove the pull off to get the vacuum line off to check. If it does not hold vacuum replace the choke pull off assembly. It corrected mine and it now runs like a champ.