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Onan twin running on one cylinder. I'm stumped...

Started by Kbeitz, July 07, 2017, 12:33:28 PM

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Kbeitz

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...



  

 
Collector and builder of many things.
Love machine shop work
and Wood work shop work
And now a saw mill work

goose63

 Kbeitz it can't be true how in the world did you ever get stumped  :D
goose
if you find your self in a deep hole stop digging
saw logs all day what do you get lots of lumber and a day older
thank you to all the vets

WV Sawmiller

   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
WM LT35HDG25(2015) , 2011 4WD F150 Ford Lariat PU, Kawasaki 650 ATV, Stihl 440 Chainsaw, homemade logging arch (w/custom built rear log dolly), JD 750 w/4' wide Bushhog brand FEL

Dad always said "You can shear a sheep a bunch of times but you can only skin him once

coxy

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

whiskers

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.........

Kbeitz

Quote from: whiskers 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.

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.
Collector and builder of many things.
Love machine shop work
and Wood work shop work
And now a saw mill work

coxy

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 

Ox

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...
K.I.S.S. - Keep It Simple Stupid
Use it up, wear it out, make it do or do without
1989 GMC 3500 4x4 diesel dump and plow truck, 1964 Oliver 1600 Industrial with Parsons loader and backhoe, 1986 Zetor 5211, Cat's Claw sharpener, single tooth setter, homemade Linn Lumber 1900 style mill, old tools

coxy

yep iv had that happen on an skidoo 399 also maybe a crank seal

JV

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

'05 Wood-mizer LT40HDG28-RA, Lucas 613 Swing Mill, Stihl 170, 260 Pro, 660, 084 w/56" Alaskan Mill, 041 w/Lewis Winch, Case 970 w/Farmi Winch, Case 850 Crawler Loader, Case 90XT Skidloader, Logrite tools

Kbeitz

Quote from: JV 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.

I checked the valve seats and the valves look to be opening up around 1/4"
Collector and builder of many things.
Love machine shop work
and Wood work shop work
And now a saw mill work

rasman57

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.

Jeff

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.
Just call me the midget doctor.
Forestry Forum Founder and Chief Cook and Bottle Washer.

Commercial circle sawmill sawyer in a past life for 25yrs.
Ezekiel 22:30

whiskers

 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.........

Kbeitz

Quote from: Jeff 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.

I changed the coil once. Maybe I need to try another.
Collector and builder of many things.
Love machine shop work
and Wood work shop work
And now a saw mill work

pineywoods

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.
1995 Wood Mizer LT 40, Liquid cooled kawasaki,homebuilt hydraulics. Homebuilt solar dry kiln.  Woodmaster 718 planner, Kubota M4700 with homemade forks and winch, stihl  028, 029, Ms390
100k bd ft club.Charter member of The Grumpy old Men

whiskers

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.........

Kbeitz

Collector and builder of many things.
Love machine shop work
and Wood work shop work
And now a saw mill work

coxy

did you try a compression test after it was hot

Jeff

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.
Just call me the midget doctor.
Forestry Forum Founder and Chief Cook and Bottle Washer.

Commercial circle sawmill sawyer in a past life for 25yrs.
Ezekiel 22:30

Kbeitz

Quote from: Jeff 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 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.
Collector and builder of many things.
Love machine shop work
and Wood work shop work
And now a saw mill work

Sawmill Man

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".

Jeff

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.
Just call me the midget doctor.
Forestry Forum Founder and Chief Cook and Bottle Washer.

Commercial circle sawmill sawyer in a past life for 25yrs.
Ezekiel 22:30

Kbeitz

Quote from: Jeff 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.

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.
Collector and builder of many things.
Love machine shop work
and Wood work shop work
And now a saw mill work

whitepine2

Quote from: Jeff 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.

   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.

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