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Old 12-14-2015, 11:22 PM   #1
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Onan generator help.

Not sure if I should put this here or mechanical. I'll start here.

I found an Onan generator on craigslist for $75. The ad said it was listed at $275 until it developed a problem staying running. I went and picked it up. We started it and it ran for about 30 seconds before dying. It could be started again. I thought it might be bad gas and maybe gummed up carburetor. The seller said he thought it might be the governer. This is old school. Any ideas where or how to diagnose the problem?
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Old 12-15-2015, 12:31 AM   #2
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Take it to an onan service shop, use a fake name/ burner phone ($20 at walmart or free with google voice). See what it needs and if it needs more than its worth just walk away from the 75 dollar problem.
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Old 12-15-2015, 12:33 AM   #3
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Take it to an onan service shop, use a fake name/ burner phone ($20 at walmart or free with google voice). See what it needs and if it needs more than its worth just walk away from the 75 dollar problem.

That's not how I roll. There are no electronics and I want to fix this myself. It can't be that hard.
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Old 12-15-2015, 06:48 AM   #4
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if it restarts within a few seconds of dying, clean entire fuel system (it needs it anyway). if it still dyes, or has to cool for awhile before restart, check the coil.

a quick check i would always do was to hold a can of wd-40 near the carb throat. when it started to wheez or die, give short burstsof wd40 to kep it running. this confirms its a fuel starvation thang.
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Old 12-15-2015, 08:22 AM   #5
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a quick check i would always do was to hold a can of wd-40 near the carb throat. when it started to wheez or die, give short burstsof wd40 to kep it running. this confirms its a fuel starvation thang.
Yes, I agree. A can of starting fluid will work well too. This will also tell you if you have any gaskets leaking which would indicate a loss of vacuum.

Take a compression check.

I had a landscape biz for twenty years and, during that time, operated literally dozens of small engines. The very FIRST thing that we always did was simply change the spark plugs. Often the the problem was not with the plug/s but just as often it WAS. Spark plugs for small engines are notoriously poor quality but cheap to replace.

I also agree that, with no electronics, the problem would simply not be that difficult to trace.

Check to see what sort of fuel pump this engine has. Many, many small engines use a vacuum fuel pump that can develop a less than optimum diaphragm if allowed to sit for an extended time. This could also be the problem.
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Old 12-15-2015, 10:07 AM   #6
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Being all mechanical, It should be easy to diagnose. Claydbal hit the nail on the head with diagnosing it. You need four things for any engine to run: Fuel, air, ignition and compression. Because it will start and run initially, odds are you're looking at fuel and ignition. So do what claydbal said, I usually use carb cleaner, I stay away from starting fluid(too volatile), and test it out.

The hard part might be trying to find parts for it.

PS. How loud is it? I see one on craigslist that looks identical, but I already have a briggs powered generator. So if the onan isn't something I can run all night and not upset the neighbors, then it's no use to me. And I'm not spending 4,000 on a generator that gets used twice a year.
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Old 12-15-2015, 10:14 AM   #7
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I will try to make a video and post it. It is a slow speed, so 1800rpm. It is definitely not a Honda EU2000 but I have heard louder. This thing weighs about 250 pounds and only rated at 2000 watts.
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Old 12-15-2015, 11:56 AM   #8
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There's experts at:

Onan Generators - SmokStak
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Old 12-15-2015, 12:07 PM   #9
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There's experts at:

Onan Generators - SmokStak

Waiting for my registration to get approved.
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Old 12-15-2015, 12:53 PM   #10
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Did you check fuel lines for any cracks no matter how small?? Gas these days just destroys rubber fuel lines.
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Old 12-15-2015, 03:59 PM   #11
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A heads up that in recent years, WD-40 uses CO2 as its propellant to make it less burnable. They used to use butane (IIRC). Not sure how this affects the WD-40 trick, though.
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Old 12-15-2015, 07:43 PM   #12
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Take the carb apart. That is most always the problem. Doubt it has anything to do with the gov. Pretty dead simple units.

Dont lose parts or rip gaskets. Stuff is getting scarce. If you do, you'll have to improvise.

What model is it?
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Old 12-15-2015, 08:47 PM   #13
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Check out the air intake all the way from the filter to the carb.
I have a similar Onan.
Last spring it ran like yours.
Acorns in the carb throat.
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Old 12-15-2015, 10:41 PM   #14
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carb rebuild should fixit.

gas that sits in carbs, turns to shellac in carbs
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Old 12-16-2015, 06:24 PM   #15
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There are relays in under the cover behind the ammeter. When you hold the "start" switch the engine will turn over. There is a relay that responds to oil pressure that keeps power to the ignition as long as there is pressure.

I seem to recall that there is also a delay relay that ignores no oil pressure for the first 30 seconds while the pressure builds. That might be on other models, it may depend on you holding the "start" switch.

Start it with the cover off of the relay box, and see if one of the relays releases just before the engine starts to die.

It may be as simple as a plugged or bad oil pressure switch.

Now that I think about this, if you hold the "start" switch continuously, does the starter crank forever, or does the oil pressure cause the relay to release the starter motor once it is running a bit?
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Old 12-16-2015, 07:03 PM   #16
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Quote:
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There are relays in under the cover behind the ammeter. When you hold the "start" switch the engine will turn over. There is a relay that responds to oil pressure that keeps power to the ignition as long as there is pressure.

I seem to recall that there is also a delay relay that ignores no oil pressure for the first 30 seconds while the pressure builds. That might be on other models, it may depend on you holding the "start" switch.

Start it with the cover off of the relay box, and see if one of the relays releases just before the engine starts to die.

It may be as simple as a plugged or bad oil pressure switch.

Now that I think about this, if you hold the "start" switch continuously, does the starter crank forever, or does the oil pressure cause the relay to release the starter motor once it is running a bit?

Interesting. It doesn't have a starter. It has an exciter starter??
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