Author Topic: Onan 4.0 BGE Surges at Idle  (Read 5663 times)

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Offline Lefty

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Onan 4.0 BGE Surges at Idle
« on: April 06, 2012, 10:13 PM »
Well, I finally got our genset to run today.... :)clap
I wouldn't say it's running properly, or even well... but it will crank and run now.
Background- It's an Onan 4.0 BGE spec:C genset in our 1988 Georgie Boy. P.O. was the grandson of the original owner, and said he hadn't known it to have been cranked by his grandfather since around '90... Judging by the 1/4" of rust/sediment in the carb fuel bowl, I'd have to agree with that estimate. It's only showing 123hrs on the hour meter.

To start with, the original Facet fuel pump was in a small pile inside the compartment where the P.O. attempted to repair it... for future reference, these pumps aren't meant to be disassembled. ???
To save money, I replaced it with a Mr. Gasket universal electric pump ($30)rated for 2.5 p.s.i. to 4 p.s.i.
Then I removed the plugs, added 1 Tbs. Marvel Mystery Oil to each cyl. and let that sit for a few days (actually was a couple months.. but don't tell anyone... :angel: ) I drained the oil, and changed the oil filter. and installed a pair of new spark plugs.
I removed the carb. and disassembled it. I cleaned it up best I could, and put it back together. (I didn't buy a carb rebuild kit.. I just cleaned the pieces in carb cleaner and put it back together).
I removed the points, and using my old points file, filed them square and reinstalled them. I gapped them to the 0.021" specified and tried to start the genset. It wouldn't fire a lick.
After some checking with my volt meter and test light, I could tell it was getting power to the points and coil, as well as to the fuel pump. The low oil pressure circuit was also ok. So I bench tested the coil and it tested fine.
I then used a test light to see if the points were breaking... the test light should blink off/on as the points break when turning the engine over... mine stayed steady on... I pulled the plugs and again reset the gap on the points... still wouldn't break. So the points had to have an internal short.
I ordered new ones from Camping World ($38.00+$5. shipping)and replaced the points and condenser with the new ones. After gapping them, I tried running the genset and it started right up. Yay!
After letting it run a while, I did what I could to get it to smooth out but it's still got a bad surge at low rpm... I sped up the idle speed until it was running as close to 1,800 rpm as I could guesstimate, and then switched he interior breakers over to genset. I ran the roof A/C off it and tested the output at the wall plugs. I'm getting between 115v-123v at the plugs with the A/C on. So at least I know it's making power... I let it run for a couple hours today with  the A/C on.
Going to remove the carb again tomorrow and do a proper rebuild on it... hopefully that will cure the surging and I can set the idle speed down to where it should be.
I reserve the right to reject your reality and substitute my own...

Online LJ-TJ

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Re: Onan 4.0 BGE Surges at Idle
« Reply #1 on: April 07, 2012, 07:00 AM »
 :)ThmbUp Good stuff. Congratulations  :)clap

Offline ClydesdaleKevin

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Re: Onan 4.0 BGE Surges at Idle
« Reply #2 on: April 07, 2012, 08:43 AM »
Awesome!  Glad you brought it back to life!

Kev
Kev and Patti, the furry kids, our 1981 Ford F-100 Custom tow vehicle, and our 1995 Itasca Suncruiser Diesel Pusher.

Offline 87Rockwood

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Re: Onan 4.0 BGE Surges at Idle
« Reply #3 on: April 07, 2012, 06:54 PM »
We just got our 4.0 BGE running 2 weeks ago and it had a surge problem when cold or in high humidity. Turned out it was the lack of the heater tube that caused it. The air heater tube stopped the carb ice from forming which caused our surge problem (plastic about 1 1/2' long)
Lynn

Offline fgutie35

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Re: Onan 4.0 BGE Surges at Idle
« Reply #4 on: April 10, 2012, 07:53 AM »
Three things you can check to smooth things up. One, check the altimeter compensator under the bowl, make sure is set for the correct altitude. Then adjust idle speed according to the manual. Then take out the spark plug wires and take them to Auto Zone or NAPA and replace them for new ones (they sell odds and ins spark plug wires). In my case, I got the genny to run 98% smooth, but still had a very faint surge. The spark plug wires replacement did the trick.

Offline tbianco

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Re: Onan 4.0 BGE Surges at Idle
« Reply #5 on: June 14, 2021, 01:07 PM »
My problem? I Don't  know where BTC is when setting points. I have 1986 onan 4.0 bfa rv genset and could really use the help

Offline Oz

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Re: Onan 4.0 BGE Surges at Idle
« Reply #6 on: June 15, 2021, 07:18 AM »
If it's an a-c spec:
21° - 25°

Offline legomybago

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Re: Onan 4.0 BGE Surges at Idle
« Reply #7 on: June 21, 2021, 01:26 PM »
lack of fuel will make them surge. You should have an adjustment petcock on the bottom of the bowl, turn it clockwise until the engine craps out, then back it off just a touch until it runs smooth. Test the gen with a load on it
Never get crap happy with a slap happy pappy