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Author Topic: Onan twin running on one cylinder. I'm stumped...  (Read 4736 times)

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Offline Kbeitz

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #20 on: July 08, 2017, 04:49:46 pm »
Did you try a known good coil, or something you had or found in the junkyard?  The coils are not cheap, and many a retired on an was retired with a bad coil.

I have a collection of engines. It was just off another engine.
I swapped the coil wires around and it makes no difference.
I got a good 1/4" blue spark off the wire.
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Offline Sawmill Man

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #21 on: July 08, 2017, 05:16:17 pm »
Try choking it while running to see if the dead cylinder will hit some. If it does then look for a leaking or cracked intake manifold. Try spraying starting fluid in places you cant see very well.
"I could have sworn I went over that one with the metal detector".

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #22 on: July 08, 2017, 05:23:27 pm »
Not trying to be funny, but did you take the intake manifold off and check it for critters on one side? Or critter stashes?  We had a quad up to the cabin years ago that quit running.  turned out there was a dead red squirrel in the airbox. It chewed through the filter.

You could have one side of the manifold obstructed.
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Offline Kbeitz

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #23 on: July 08, 2017, 07:35:11 pm »
Not trying to be funny, but did you take the intake manifold off and check it for critters on one side? Or critter stashes?  We had a quad up to the cabin years ago that quit running.  turned out there was a dead red squirrel in the airbox. It chewed through the filter.

You could have one side of the manifold obstructed.

Nope the intake manifold has not been off yet. I did spray starting fluid all over
it looking for leaks. I also sprayed starting fluid down the carb and seen no difference.
I'm thinking that I must have a broken valve spring. The valves are working and I get
110 psi when checking but I'm thinking when the engine is running the the spring is
not strong enough to close the valve. I don't know what else it could be. This engine has a good air cleaner on it and I don't know how a critters could get down the carb past the filter.
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Offline whitepine2

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #24 on: July 08, 2017, 07:42:57 pm »
Not trying to be funny, but did you take the intake manifold off and check it for critters on one side? Or critter stashes?  We had a quad up to the cabin years ago that quit running.  turned out there was a dead red squirrel in the airbox. It chewed through the filter.

You could have one side of the manifold obstructed.
 
   This is what I was thinking,had a field mouse eat into my air breather on four
cylinder diesel it would start but not run fast. I was lucky he or his family didn't quite
make it to the turbo one did climb out and into the fan. What a mess as they chewed
up the filter and it got sucked into the outlet just letting enough air in to start. Could
have bee's nest in there or may-be a rag who knows,I'm sure you will figure it out
  Good luck    W.C.W.

Offline Kbeitz

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #25 on: July 08, 2017, 08:38:40 pm »
My air cleaner filter would need to have a hole in it for a mouse or rag to get past it.
But you are right it seem like one side is not getting any gas.
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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #26 on: July 08, 2017, 10:38:26 pm »
You need to confirm that air fuel can get down that side. Mice and bees can get in a hole not much bigger that a pencil eraser.
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Offline Kbeitz

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #27 on: August 13, 2017, 08:53:28 pm »
I bought another compression checker. This new one I got says I got 100 psi on the running side
and 95 psi on the non-running side. So just for the heck of it I took the top of the engine off and
backed off the tappets on the non-running side one turn. Now it will run on both side at full throttle
but nothing lower. I'm busy tomorrow but Tuesday I'm going to back off the tappets another turn
and see what happens. They are suposta be set at IN: 0.005" and EX: 0.013"  but that's not working.
I'm still puzzled. I drove it around for about 30min to see if it would change. Nope...
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Offline slider

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #28 on: August 14, 2017, 08:48:06 am »
Kebitz this may not help but i had a similar problem with a 24 hp onan and it was trash in the main jet.Under load it would stumble and seamed to be running on one cylinder.Jeff is right about the coils the oem coils were not cheap.
al glenn

Offline Kbeitz

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #29 on: August 15, 2017, 10:04:44 am »
I have tried different coils and I switches the wires from one side of the coil to the other with no difference.
I'm going to pull an intake and car off another engine and try that.
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Offline whitepine2

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #30 on: August 26, 2017, 09:53:29 pm »
  Did you ever find out what was going on with your one cylinder firing?     

Offline ljohnsaw

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #31 on: August 26, 2017, 11:41:19 pm »
Also waiting for a solution.  I had a mouse family move in to my air box on my Grizzly 550 - what a mess...

Unrelated but interesting (to me).  I have a 16hp Wisconsin on my trencher.  Was running great last year and earlier this spring but just couldn't get it to start a few weeks ago.  No spark.  I had put in new points and coil along with a new carb last year.  I opened up the points cover (could not see into it with the engine installed in the trencher).  I scraped a knife through the contacts (corrosion?) and now it runs great again.  I did not feel any roughness when scraping, though.
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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #32 on: August 27, 2017, 03:21:35 am »
No... It's sitting in line to be worked on. I have other onan engines for parts.
I guess I got to do the shade tree mechanic thing and start swapping parts
and see what works. First thing I'm trying is to switch the intake and carb.
The tractor is in very good condition so I don't want to give up on her.
This has been a very busy year for me. After I finish my edger I have a can
conveyor I need to build for the junkyard so I will be awhile until I can get
to the tractor, but I will keep you all posted. Thanks for your interest.
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Offline Ox

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #33 on: August 27, 2017, 10:22:54 am »
I'm also waiting.  I've never heard of such a thing and want to see what caused this problem.

What is a can conveyor?

I don't know how this will matter, but a backfire can sometimes half shear the key off holding the flywheel which can throw the timing off.  But since the other cylinder is firing this don't make any sense.  Just figured I'd throw it out there to maybe spark a thought or something.

It may be a slightly bent valve stem or something like that.  I've seen the ethanol fuel actually stick valves in the guide and then they bend.  This happened after sitting over the winter mostly I think.



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Offline coxy

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #34 on: August 27, 2017, 11:10:06 am »
I'm sticking to what I said before of it being a week cylinder and the hotter it gets the less compression it has to fire  :)

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #35 on: August 27, 2017, 11:40:24 am »
It makes sense.
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Offline Kbeitz

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #36 on: August 28, 2017, 07:32:04 pm »
I'm sticking to what I said before of it being a week cylinder and the hotter it gets the less compression it has to fire  :)

Every time I check it it has at least 100 psi or more.
Is that not enough?
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Offline Kbeitz

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #37 on: August 28, 2017, 07:36:51 pm »
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Offline hamish

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #38 on: August 28, 2017, 07:56:59 pm »
Google say an engine will run as low as 45psi

https://www.lawnsite.com/threads/compression-test-for-small-engines.72249/

A four stroke engine will not run with a mere 45psi of compression, regardless of what Mr Google says.
Even engines with an automatic compression release when run in there normal orientation will not run with 45 PSI Compression.
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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #39 on: August 28, 2017, 08:38:52 pm »
I've got an old b&s 3.5 hp push mower with so much blow by from the rings that it will choke itself out if the breather line is hooked to the carb. It will however start on the 1st or 2nd pull after sitting all winter sith the same old gas in it. I've been waiting for the thing to die for the past 8 years but it keeps on starting every time.
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