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Author Topic: Onan twin running on one cylinder. I'm stumped...  (Read 6627 times)

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Offline coxy

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #40 on: August 28, 2017, 08:59:47 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?
are you checking it after its hot  the Honda motor I had tested good till you run it for a half hour or more at full throttle then would loose compression on one side and run like crap  I'm not saying that's whats wrong with yours

Offline Kbeitz

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #41 on: August 29, 2017, 06:54:35 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.

I found more links that say you can make an engine run with 30 psi.
Others say above 75 pounds then the engine has enough compression to run.

I know my old Hit&Miss engines are very low compression engines.

But from everything I read I think my 100 psi should be running.
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Offline starmac

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #42 on: September 10, 2017, 12:54:10 AM »
Probably is not your problem, but I have several times had new plugs that would throw a pretty spark, but not fire a lick under compression, this has usually been with ngk plugs, but seen the same thing with other brands too.

9 times out of 10, I could pull the plug and spray it with WD40 and it would crank at least once. I do not know the compression the onan should put out, but it should at least hit on 100.
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Offline martyinmi

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #43 on: September 10, 2017, 06:49:15 AM »
Just for giggles, back off your intake lash to .020" and exhaust to .030".
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Offline Kbeitz

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #44 on: September 10, 2017, 11:31:04 AM »
Just for giggles, back off your intake lash to .020" and exhaust to .030".

Thanks... I will try that...
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Offline Kbeitz

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #45 on: September 17, 2017, 06:44:25 PM »
Well I put a few more hours working on it and still nothing.
I changed out the carb and intake. It made no difference at all.

 

 

 

 

 

 
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Offline Andries

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #46 on: September 17, 2017, 06:56:26 PM »
My LT30 has a 24 hp Onan, which had the same problems that you describe.
After a whole lot of head scratching, it came back to reply #19.
Replaced the old coil with aproven and new coil.
That was five years ago.
She runs like a champ now.
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Offline Kbeitz

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #47 on: September 17, 2017, 08:36:32 PM »
My LT30 has a 24 hp Onan, which had the same problems that you describe.
After a whole lot of head scratching, it came back to reply #19.
Replaced the old coil with aproven and new coil.
That was five years ago.
She runs like a champ now.

Tomorrow I will swap a coil off one of my other running tractor and report back.

Thanks for all the ideas..
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Offline coxy

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #48 on: September 18, 2017, 06:47:38 AM »
 popcorn_smiley popcorn_smiley

Offline elitts

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #49 on: September 19, 2017, 08:56:33 AM »
If you don't end up finding a solution, I would suggest taking your question to mytractorforum and asking it under the "Lawn & Garden Tractor"/Gravely forum.  The old 1970-1990 Gravely tractors frequently used Onan 16hp & 18hp engines and there are a couple of guys there who really REALLY love Onan engines and have spent decades restoring them.

Offline Kbeitz

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #50 on: September 19, 2017, 08:59:26 AM »
If you don't end up finding a solution, I would suggest taking your question to mytractorforum and asking it under the "Lawn & Garden Tractor"/Gravely forum.  The old 1970-1990 Gravely tractors frequently used Onan 16hp & 18hp engines and there are a couple of guys there who really REALLY love Onan engines and have spent decades restoring them.

Thanks... I'm a member there. I love that forum until they changed everything.
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Offline starmac

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #51 on: September 20, 2017, 11:43:05 PM »
If there is no vacuum leak and the dead cylinder has compression and is getting gas, it sure seems like the coil should be the culprit, but then again switching the plug wire should have changed the dead cylinder too. So far this is a head scratcher.
Old LT40HD, old log truck, old MM forklift, and several huskies.

Offline Kbeitz

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #52 on: September 21, 2017, 12:46:30 AM »
If there is no vacuum leak and the dead cylinder has compression and is getting gas, it sure seems like the coil should be the culprit, but then again switching the plug wire should have changed the dead cylinder too. So far this is a head scratcher.

Sorry I just did not get time yet to try a third coil change.

I had the same thoughts as above. But then I remembered that
even though I swapped the wires in the coil and tried new wires
the wire going to the front cly that's not working is around  6-8"
longer that the one that's working. It shows a real good spark
when the plugs out but things change when under compression.
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Offline starmac

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #53 on: September 21, 2017, 02:20:23 AM »
Well I will be the first to admit that I have not seen everything (or even close to it). I have seen plugs that fired good when grounded, but not under compression several times, but never a coil. I do not see how compression could possibly affect the spark coming from the coil.

If the plug wire on the affected cylinder is longer, then you probably changed the wires at the coil itself, which makes me wonder if their might not be too much resistance in the wire itself.
Old LT40HD, old log truck, old MM forklift, and several huskies.

Offline Al_Smith

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #54 on: September 21, 2017, 06:21:02 AM »
Just for general info those Onan coils on some models are a double coil like a Harley .Both plugs fire at the same time .

Offline Kbeitz

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #55 on: September 21, 2017, 06:33:35 AM »
All coil wires that I changed came off other Onans engines.

Yes this has a double coil just like a Harley.

It's called a wasted spark system.
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Offline Hilltop366

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #56 on: September 21, 2017, 11:46:00 AM »
Kind of sounds like a intake vacuum leak or valve problem to me.

If it was a no spark issue I would think the spark plug on the dead cylinder would be wet after running.

Offline Kbeitz

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #57 on: September 21, 2017, 08:25:14 PM »
Kind of sounds like a intake vacuum leak or valve problem to me.

If it was a no spark issue I would think the spark plug on the dead cylinder would be wet after running.

Changed the intake twice and it has a little over 100psi on the no running side.
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Offline coxy

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #58 on: September 21, 2017, 08:28:53 PM »
are you checking it after it has run for a little while and getting a 100

Offline Magicman

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Re: Onan twin running on one cylinder. I'm stumped...
« Reply #59 on: September 21, 2017, 08:46:53 PM »
How are the valves timed?
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