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Old 06/04/08, 10:30 AM
Join Date: Jun 2007
Posts: 1,064
Diagnosing generator problem

My well-pump is powered by an old (1970's?) Onan generator that I enherited from the previous owner of the property. It has performed just fine for me over the last 4 years. I have kept up with oil changes, new plugs/filters, and kept a written log of run times.

After overwintering, I was getting ready to perform spring maintainance and found the generator inoperative. Using new gasoline, I can start the engine (electric start) just fine, but as soon as a load is applied the engine stalls. It will stall if a heavy 240V load is applied (well pump), or with a relatively light 120V load (circular saw). The generator does produce enough electricity to start the saw blade turning, but still stalls the engine within a second or so.

Can anyone give me suggestions for diagnosing the problem? This generator is old enough that I suspect the repair bill will be greater than going out and buying a new generator. It seems the best options are either repair it myself, or just replace it with a new unit.
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Old 06/04/08, 01:41 PM
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Join Date: Nov 2004
Location: Delaware
Posts: 2,284
The carb may have gotten a little varnishy during the winter and needs to be cleaned.
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Old 06/04/08, 04:10 PM
Join Date: May 2003
Location: Zone 7
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Check the gas supply at the carb. Trash in the outlet of the fuel tank or delivery lines to the carb or a clogged carb fuel jet will starve the engine of gas thus robbing the engine of power to perform the task.
If they can do it,
you know you can!
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Old 06/04/08, 04:16 PM
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Join Date: Jun 2002
Location: central idaho republic
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also check the fuel line, a pinhole will suck air and rob fuel too....

I put some pri-G in my gas this spring and added that to my lawnmower, the stuff after a couple tanks done washed the gunk off the sides of the tank and clogged the fuel line so it needed cleaning..... dont add a fuel additive and you lose the fuel, add it and it cleans the junk off the tank and clogs up anyway, a fella cant win for losing at times

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Old 06/04/08, 04:20 PM
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Join Date: Feb 2006
Location: Indiana
Posts: 940
How many cylinders?

If it was mine I would pull all the sparkplugs out.. Then do a compression test.It might have a stuck valve..
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Old 06/04/08, 04:34 PM
Join Date: Oct 2006
Location: Arkansas
Posts: 11,515
It sounds like the engine is fine and the generatior is staling the engine. See if theeir is a way that it is getting a shourt on the line like something before the breakers and past the generatio. If not then I would take it to a regular generatior shop not to a engine shop and let them check it out. Check out generatior shops in your yellow pages. If you don't have one around PM me and I will put you int contact with one in Arkansas and you can use them or they may find a place close to you.
God must have loved stupid people because he made so many of them.
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Old 06/04/08, 09:20 PM
Join Date: Mar 2005
Location: Colorado
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check the voltage. have mice got in it?
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Old 06/09/08, 12:21 PM
Join Date: Jan 2005
Location: NC
Posts: 635
start the engine and open the throttle wide open manually. If it stalls, you can suspect it is starving for fuel. If it accelerates nicely, then you can suspect it has to do with an electrical problem.
If you determine it is an electrical problem, hook up a timing light, start the engine, then apply an electrical load to the generator while watching the strobing of the timing light. If it quits strobing, then you know the failure has to do with robbing the ignition system of the electricity necessary to produce sparks.
Keep asking questions and figuring out how to answer them conclusively.


By the way. i love those engine generator combos. I bebuilt one for a friend. He got it out of a Winnebago form the 1970's. This one had an electric starter, but there was no separate starter on it. The battery power was used to turn the generator which started the engine. that's so cool.
The mass of the generator makes them good candidates for easily starting by a hand cranking method or a recoiling pull cord type starter, too...lots of inertia
I had a hard time finding parts for it. It turns out that the piston rings I needed were the same size as the rings used on some certain vintage Harley engine used in sportsters, if my memory serves me well...
We mounted it on a modified boat trailer and gave it a more extensive exhaust system so it would be even quieter than it already was.
that old generator has been working quietly and tirelessly for him and his parents during power outages after hurricanes for tha past 10 years or so.
He says its good on gas, too.
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Old 06/09/08, 12:28 PM
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Location: SW VA
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It could be a malfunction of the voltage regulator, but I don't know much about regulators on generators that small.
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Old 06/10/08, 10:10 AM
Join Date: Jun 2007
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Appears to be fuel related. I pulled the air intake off the carburetor and held the butterfly valve closed (choke?) and the generator started. Releasing the valve and it stalled again. After talking with a generator sales person over the phone, the conclusion was that overwintering fuel must have varnished up the carburetor and the engine is starved for fuel.

A local service guy wants 215$ just to look at it, so I've decided to just buy a new generator. I'll stick the old generator in a corner for now, and try disassembling the carburetor to clean it later. If I can get it running, it can serve as a backup, or a generator operating in a separate location. In the mean time I can look around to find a service manual for Onan generators.
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Old 06/10/08, 04:06 PM
Join Date: May 2003
Location: Zone 7
Posts: 10,837
Remove all the old gas from the generator engine. Go to an auto supply store and buy a large can of their best carb cleaner. Take the air cleaner off the carb and excessively douse the carb on the inside with the carb cleaner. Put the air cleaner back on the carb and do nothing else to the engine for a couple of days. Put some gas additive in the new gas and fill the gas tank. Pull the choke and turn the engine over a few seconds with the starter with the choke on and do not really attempt to get the engine running. Put the choke back off and let the engine set for another 24 hours or more. Return and start the engine as per the old normal. I will wager that it will run as for the old normal.
If they can do it,
you know you can!

Last edited by agmantoo; 06/10/08 at 04:08 PM.
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Old 06/11/08, 10:41 AM
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Did you try adding carb cleaner to your tank of fresh gas?

It's not good enough that we do our best; sometimes we have to do what's required. - Winston Churchill
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Old 06/16/08, 11:43 PM
Join Date: Jun 2007
Posts: 1,064
Yay, I fixed it. I disassembled the carburetor and flushed out all the ports and needle valve with carb-out cleaning spray. I also replaced the fuel line (it was so old it broke) and put in a new fuel filter. While at it I brushed up the plugs and also replaced the air filter. After priming with new gas it started right up. After warming up for a few minutes I did some load testing. The first load I put on it was 1000w (our toaster). Then I tested 2000w (the toaster and a clothes iron). So, it put out 2kw with no problem, so I'd expect it to be good to go. Lastly, after shutting it down, I changed the oil too. Now I just have to haul it back up the mountain and get it back off the back of the truck.

Thanks for all your advice,
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Old 06/17/08, 12:23 AM
Join Date: May 2003
Location: Zone 7
Posts: 10,837
And the amount of our wager was?
If they can do it,
you know you can!
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Old 06/27/08, 09:44 AM
Join Date: Dec 2005
Location: upper michigan
Posts: 120

Sounds Like Ur Gen.brushes Need Changing
Very Easy To Do
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Then loved for being someone else
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Old 07/05/08, 10:01 PM
Join Date: Jun 2007
Posts: 1,064
I think you missed the reply where I said I fixed it. After cleaning the carb and hauling it back up to the cabin I was able to use it to pump about 4000 gallons of water. Everything is back to normal now and my water worries are over.
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