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Old 04-03-2016, 12:57 PM   #1
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Onan 5500 Marquis HGJAB-901A problem

Hi All,
I would appreciate any help you could give before I start throwing parts at this genset. A little background... Just bought 02 Winnebago brave with this genset in it and before buying I was checking generator and started right up and ran like a champ for 2-3 minutes then started popping and coughing and kept dying. Never got chance to check under load. Let it set for 1/2 hr, start up and repeated same cycle. So seller called his guy and he came over and put new fuel pump on and same results. His guy was checking things with a meter and found that ATS box was feeding power back to generator. Said he never saw that before so seller and him were suspect of ATS BOX so he replaced. Fixed the feedback issue but genset still won't stay running. We negotiated price down more and I'm the proud owner. I'm mechanical but not electrical. So need some help please. His guy seemed knowledgeable but short on time. He was suspect of coils, or carb. Any ideas? Thank you very much.
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Old 04-03-2016, 02:43 PM   #2
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If it hasn't been exercised for an hour or so every month the carb can gum up and yield the symptoms you describe. Try running it while holding the choke closed. If it runs -even poorly - you may be able to get it to clear by doing that for a while. Or you may want to disconnect the fuel line from the filter and add a hose to a can of fuel with some SeaFoam in the gas and run it from that.
Onan does not sell parts for their carbs any longer (as far as I know), but you can buy a replacement - not cheap though.

Air leakage in the fuel line can also cause the problem.
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Old 04-03-2016, 03:43 PM   #3
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Clay L- thanks for reply. It definitely will keep running rough if I mess with choke. But it runs so smooth for first couple minutes though. In my mind, if it runs so good at first, how could it be gummed up? Stranger things have happened though huh?
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Old 04-04-2016, 08:09 AM   #4
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Might be good to determine if it's a fuel problem or electrical,,, If you have an old timing strobe light put it on a plug wire and watch to see if the lamp fires continuously and what it shows when the engine starts to fail.
Trying some fuel additive is not a bad suggestion. If you have the room try installing a "clear" fuel filter in line so you can see if there is starvation.

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Old 04-04-2016, 08:19 AM   #5
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fuel filter, Fuel Filter FUEL FILTER! Also check for a crimped or deteriorated fuel line. A good way to totally isolate your fuel delivery would be to remove the fuel line from the fuel pump, extend it to a fuel can (at the same level as the fuel tank, NOT on the ground) and see if that corrects your problem. If it does, I would suspect fuel filter, fuel lines, or possibly a filter/screen in the fuel tank . . . Did I mention FUEL FILTER?
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Old 04-04-2016, 09:19 AM   #6
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Scot, do you think it could be a fuel filter issue ? :>)

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Old 04-04-2016, 10:51 AM   #7
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Thanks for these great suggestions. I will try them all as soon as this new snow melts off. I really appreciate the help. I will report back.
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Old 04-04-2016, 12:08 PM   #8
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When we bought our 07 Georgetown with a 5500 onan, the seller only ran it for about a minute to show us it ran good. Little did I know, that as soon as it started to warm up and the choke started to "pull off" that it didn't want to run at all. Took me 3 times of removing and cleaning that carb to get it to run. Onan will tell you it is not a serviceable carb. Trust me....it can be pulled apart to clean.

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Old 04-04-2016, 05:49 PM   #9
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Will definitely pull that carb off when the snow melts and take the seafoam and air compressor to it before I shell out $250 for new one. And new fuel filter of course.
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Old 04-05-2016, 07:55 AM   #10
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Quote:
Originally Posted by clifford j View Post
Scot, do you think it could be a fuel filter issue ? :>)

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Cliff, that MIGHT be a possibility, but it's still too early to tell!
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Old 04-05-2016, 12:43 PM   #11
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Quote:
Originally Posted by Hit_the_Rhod View Post
fuel filter, Fuel Filter FUEL FILTER! Also check for a crimped or deteriorated fuel line. A good way to totally isolate your fuel delivery would be to remove the fuel line from the fuel pump, extend it to a fuel can (at the same level as the fuel tank, NOT on the ground) and see if that corrects your problem. If it does, I would suspect fuel filter, fuel lines, or possibly a filter/screen in the fuel tank . . . Did I mention FUEL FILTER?
I agree with Scot about the fuel filter. Be aware that an Onan of that vintage probably has TWO filters. There will be one attached to the fuel pump and another screwed into an L fitting next to the carburetor. If it has set for a long time either one or both could be gummed up. Most have replaced the filter by the carburetor with a barbed hose fitting and a longer fuel line, just keeping the one filter by the pump. The newer Onans come this way with just one filter. Another possibility is the float valve in the carburetor could be sticking and allowing it to run until the fuel in the bowl is out. I had the same generator in our old Itasca and had fuel issues with it.
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Old 04-05-2016, 09:22 PM   #12
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If your gennie runs on choke and starts acting up as the choke starts to pull off it is only one thing. The main carb circuits are gummed up or blocked. The choke circuits provides the fuel that the main one cant because its blocked. Pull apart and degum and use a very small wire to open all circuits. We see this every day at our small engine shop. A bottle of stable or lucas at storage time is a lot cheaper than a carb rebuild.
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Old 04-05-2016, 10:41 PM   #13
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If your gennie runs on choke and starts acting up as the choke starts to pull off it is only one thing. The main carb circuits are gummed up or blocked. The choke circuits provides the fuel that the main one cant because its blocked. Pull apart and degum and use a very small wire to open all circuits. We see this every day at our small engine shop. A bottle of stable or lucas at storage time is a lot cheaper than a carb rebuild.
Agree. If it runs on choke, it's getting gas thru the pump and filter.

The choke opens as the engine warms up.

Partially clogged passages will lean it out. Closing the choke makes it rich again.
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Old 04-05-2016, 11:19 PM   #14
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Once a year I add 5 cans of Seafoam to my fuel tank. I run the generator an hour with that mixture and so far haven't had any clogging issues.

Here's the manual:

http://power.cummins.com/sites/default/files/literature/rv/F-1123-EN.pdf
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