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Author Topic: Onan twin running on one cylinder. I'm stumped...  (Read 5404 times)

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Online Kbeitz

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Onan twin running on one cylinder. I'm stumped...
« on: July 07, 2017, 12:33:28 PM »
I'm really hoping someone here with more expertise than me
can shed some light on a problem I'm having.
I have a 18hp Onan twin that's only running on one cylinder.
I have worked on small engines most of my life but this one
has got me stumped.

My engine started running intermediately on
and off on one cylinder. Then it dropped the
front cylinder completely. I can not get it to
fire on that cylinder no matter what I do.

Twin Onan's fire both cylinders at the same time.
The coil fires twice per cylinder cycle: once during
the compression stroke, and once during the exhaust
stroke (as the opposing cylinder comes to compression),
a "wasted spark" system.
It has  a true coil (not a magneto), with two plug wire towers.
The coil only has one set of windings in it, it either works on
both towers , or it doesn't work at all.
I can pull the plug wire off the dead cylinder and hear no
difference in the engine running but if I pull the plug wire
off the other cylinder it dies immediately.
I can hold my hand on the the exhaust thats not working
and it's only a little warm. The other one is HOT.
Onan twins only have one carburetor for both cylinders.

Here's my diagnostic so far:

Replaced plugs and wires.
Gaped the plugs to spec.
Bright blue sparks on both.
The points and condenser were replaced.
Checked compression 110psi.
I checked the Intake manifold gasket for
leaking on the dead cylinder.
I remover the head to check the gasket
and valves. Everything looks good.

I'm lost...

 

  

 
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Offline goose63

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #1 on: July 07, 2017, 04:15:13 PM »
 Kbeitz it can't be true how in the world did you ever get stumped  :D
goose
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Offline WV Sawmiller

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #2 on: July 07, 2017, 04:26:47 PM »
   I'm with you, Goose. I figured if Kbeitz had a problem with an engine he could not fix he'd better get a Priest and some Holy Water.
Howard Green
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Offline coxy

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #3 on: July 07, 2017, 04:36:09 PM »
I have a 20hp Honda twin that would not fire on one cylinder after it run a little come to find out it was bad rings after rebuilding it runs fine not saying that's your trouble but that's what happens on a Honda  would never do it cold

Offline whiskers

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #4 on: July 07, 2017, 04:50:35 PM »
you have compression and fire that leaves fuel air and timing. the dead cylinder plug after cranking without the plug wire comes out wet or dry. dry plug =no fuel, wet plug insufficient air or incorrect timing. just wondering if a valve isn't opening enough.
many irons in the fire.........

Online Kbeitz

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #5 on: July 07, 2017, 05:12:17 PM »
you have compression and fire that leaves fuel air and timing. the dead cylinder plug after cranking without the plug wire comes out wet or dry. dry plug =no fuel, wet plug insufficient air or incorrect timing. just wondering if a valve isn't opening enough.

Just by looking I would say the valves are opening around 1/4"
I'm thinking that it's not getting gas. But I sprayed starting fluid all
around the intake looking for a leak and I found none. With one carb
how could it not be getting gas. I don't think the timing could change
on only one piston and not the other. Should run with 110 lbs compression.
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Offline coxy

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #6 on: July 07, 2017, 05:47:08 PM »
what is the com test # after its hot when mine was cold it had good com but after it was hot it would loose70- 90lbs depending on how long you ran it 

Offline Ox

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #7 on: July 07, 2017, 05:48:26 PM »
Worn camshaft?  What about a compression release system failure?  On a Kohler K321 when the compression release system fails it won't build compression until it's fixed.  I can't remember how the Onans are or even if they have a compression release.

I had a Suzuki 230 Quadrunner give me fits years ago.  Brand new plug, bright blue spark but never would run.  Gas getting in, good compression, etc. etc. etc.  Turns out that plug wasn't firing when it was installed and under compression.  How that happens I'll never know but a new plug from a different manufacturer had it running immediately.  Beats me.  But it might give you something to try.  Switch plugs around from one cylinder to the other.  Maybe swap plug wires around.  Just throwing ideas...
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Offline coxy

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #8 on: July 07, 2017, 05:53:49 PM »
yep iv had that happen on an skidoo 399 also maybe a crank seal

Offline JV

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #9 on: July 07, 2017, 06:10:45 PM »
I'm certainly no expert but I understand that some Onans were prone to having valve seats loosen.  Also the cam bearings could spin out.  My 1/2 cents worth.
John

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Online Kbeitz

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #10 on: July 07, 2017, 06:21:26 PM »
I'm certainly no expert but I understand that some Onans were prone to having valve seats loosen.  Also the cam bearings could spin out.  My 1/2 cents worth.

I checked the valve seats and the valves look to be opening up around 1/4"
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Offline rasman57

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #11 on: July 07, 2017, 07:00:48 PM »
You mentioned the bad cylinder went intermittent and not a sudden failure so that at least helps with narrowing the range of consideration.    Electrical usually is all or nothing but mechanical failure.....cracks, leaks, timing  etc can develop and then failure.   If spark and fuel are really making it to that cylinder I would make sure the timing of the spark is correct for close to TDC on that cylinder and the valves sequence matches intake- compression-power-exhaust.

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #12 on: July 07, 2017, 07:04:12 PM »
Its the coil. Search my posts using Onan coil. I had the exact same problem, and thought the same thing about running or not running. It was the coil.
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Offline whiskers

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #13 on: July 07, 2017, 07:05:37 PM »
 remove the good cylinder plug wire.
 prime the dead cylinder with a bit of gas replace it's plug and wire.
 spin over, if it putts check the intake and exhaust port for carbon build up.
many irons in the fire.........

Online Kbeitz

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #14 on: July 07, 2017, 09:20:42 PM »
Its the coil. Search my posts using Onan coil. I had the exact same problem, and thought the same thing about running or not running. It was the coil.

I changed the coil once. Maybe I need to try another.
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Offline pineywoods

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #15 on: July 07, 2017, 10:09:53 PM »
K, swap the plug wires, see if the problem moves to the other cylinder. If it does, it's the coil. Pulling one wire off will kill both cylinders. Onan ignition is weird. The secondary winding is isolated. One end of the winding goes to one plug, the other end of the winding goes to the other plug. Kinda hard to get your mind around it.
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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #16 on: July 08, 2017, 12:09:35 PM »
mine runs rough on both cylinders, it runs rougher with one wire removed. not sure if has the original coil.   '74 FLH  Rolls Canardly with after market coil and point ignition is the same.
many irons in the fire.........

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #17 on: July 08, 2017, 12:40:27 PM »
I changed coils again... Still no change...
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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #18 on: July 08, 2017, 02:04:24 PM »
did you try a compression test after it was hot

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #19 on: July 08, 2017, 04:47:26 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 onan was retired with a bad coil.
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