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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Kbeitz it can't be true how in the world did you ever get stumped :D
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.
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
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.
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.
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
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...
yep iv had that happen on an skidoo 399 also maybe a crank seal
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.
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"
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.
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.
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.
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.
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.
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.
I changed coils again... Still no change...
did you try a compression test after it was hot
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.
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.
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.
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.
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.
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.
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.
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.
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...
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.
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.
Did you ever find out what was going on with your one cylinder firing?
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.
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.
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.
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 :)
It makes sense.
Quote from: coxy 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 :)
Every time I check it it has at least 100 psi or more.
Is that not enough?
Google say an engine will run as low as 45psi
https://www.lawnsite.com/threads/compression-test-for-small-engines.72249/
Quote from: Kbeitz on August 28, 2017, 07:36:51 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'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.
Quote from: Kbeitz on August 28, 2017, 07:32:04 PM
Quote from: coxy 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 :)
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
Quote from: hamish on August 28, 2017, 07:56:59 PM
Quote from: Kbeitz on August 28, 2017, 07:36:51 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.
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.
Just for giggles, back off your intake lash to .020" and exhaust to .030".
Quote from: martyinmi on September 10, 2017, 06:49:15 AM
Just for giggles, back off your intake lash to .020" and exhaust to .030".
Thanks... I will try that...
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.
(https://forestryforum.com/gallery/albums/userpics/39553/DSC06730.JPG?easyrotate_cache=1505688168)
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(https://forestryforum.com/gallery/albums/userpics/39553/DSC06732.JPG?easyrotate_cache=1505688256)
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.
Quote from: Andries 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.
Tomorrow I will swap a coil off one of my other running tractor and report back.
Thanks for all the ideas..
popcorn_smiley popcorn_smiley
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.
Quote from: elitts 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.
Thanks... I'm a member there. I love that forum until they changed everything.
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.
Quote from: starmac 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.
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.
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.
Just for general info those Onan coils on some models are a double coil like a Harley .Both plugs fire at the same time .
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.
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.
Quote from: Hilltop366 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.
Changed the intake twice and it has a little over 100psi on the no running side.
are you checking it after it has run for a little while and getting a 100
How are the valves timed?
I don't think them onans had hydraulic lifters, if so, maybe collapsed lifter ? Way back in the dim dark past, seems I remember getting bit by a similar problem on a briggs. One lobe on the camshaft was completely worn off, intake valve never opened...
I was not considering hydraulic lifters but even so, whether the valves were lifting and at the proper time.
If that were the case neither side would run. Did you have the valves out and check for a loose seat?
Yup, you are correct regarding the timing. Kb has made compression test which are OK, the spark is OK, carburetor is OK, so the only thing left is the gas mixture is not getting into the cylinder, or the exhaust is never leaving.
It doesn't make any sense it would fire one side of the coil and not the other .Assuming it's a battery spark which frankly is all I've
ever seen on that engine it only has a set of points and condenser plus the double coil .
Actually on that "wasted spark " it isn't really wasted .By firing every time the piston is near top dead center it has a tendency to keep the plug clean .
The only thing I can think off would be the coil halves being wired in series which I doubt would be the case .Perhaps a weak condenser with the first coil in line robbing all the voltage but that is highly unlikely .
Rambling on I once had a mag coil go funky on an Eisenman mag on a D4 Cat pony engine .Temporarily to get it running I used the mag points and two car coils and a 6 volt dry cell lantern battery .If you collect junk some times you have to get rather ingenious . :D
Quote from: Magicman on September 22, 2017, 08:36:30 AM
Yup, you are correct regarding the timing. Kb has made compression test which are OK, the spark is OK, carburetor is OK, so the only thing left is the gas mixture is not getting into the cylinder, or the exhaust is never leaving.
Just thinking out loud, if the exhaust valve is not opening, I would expect to see some back pressure in the carb. With the air filter off, see if you can feel puffs of air when it is running on the good cylinder. Or maybe turn it over by hand and see what kind of pressure you get if you cover the carb with the palm of your hand. May be able to tell if pressure is coming back from the intake (not being relieved by the exhaust valve)??
I'm about to give it up on this engine. I spent another 3 hours switching plugs, coil and wires.
I pulled the head again to check the working of the valves and to see if the head gasket was
still good. Then i pulled the valves and reground them and set to specs. Still nothing. It
wont even try to fire on one side.
(https://forestryforum.com/gallery/albums/userpics/39553/Dsc06796.jpg?easyrotate_cache=1506125760)
(https://forestryforum.com/gallery/albums/userpics/39553/DSC06797.JPG?easyrotate_cache=1506125871)
(https://forestryforum.com/gallery/albums/userpics/39553/DSC06798.JPG?easyrotate_cache=1506125934)
Tomorrow I'm changing the point and condenser one more time and then I'm done.
If it does not run I quite.
O no you can't quit not after all you have gone through ya gotta make it go and let us know
just what was wrong,you gotta! :P :P :P 8) 8) 8)
Well you certainly can't take it to the Junkyard. :-X
Quote from: Magicman on September 22, 2017, 09:36:58 PM
Well you certainly can't take it to the Junkyard. :-X
Nope can't do that,it's probably something so simple sometimes ya just can't see
the forest for the trees. I have a feeling K isn't going to give up ether it's not in his
DNA.
Compression is now the same 105 lbs both sides.
Ran extra hot wire to coil.
Put battery charger on battery for more amps.
Both spark plugs was new. Again
All swapped parts came off another running tractor this time.
carb
intake
coil
valve springs
coil wires
points
head gasket
Whats left to do ???
It looks to have very good spark both sides.
Dumped extra gas down the carb when running.
Valves are working.
Compression good.
Timing cant change only on one side.
One set of points
One carb.
One coil.
I even made my own gas engine from scratch and I had no problem getting it to run.
(https://forestryforum.com/gallery/albums/userpics/39553/compressor_engine~0.jpg?easyrotate_cache=1506137246)
I even had help the last time by someone that works on engines...
He had no clue...
don't get mad at me but try a set of rings what do you have to loose but a few more hours and a 100 bucks if that don't work ill never give you advice again :-X :D :D
I would still buy a new coil. Years ago I had a dirt bike that quit,I changed the spark plug with my buddy's bike. My bad plug worked fine in his, but would'nt in mine. So even though you put one from a running motor, it might be something to consider,especially instead of giving up on it.
Ayup.
I think Onan quit selling twin coils years ago or I would have a new one.
Kohler still makes one. Maybe I'll try one of there's .
As you say swapping the wires should make the problem switch sides, but it does not. That leads me to ask, does this model have an ignition trigger? Seems I read that it has a two circuit system, one side could be bad ? Be careful testing that, as they say that they are easily destroyed. First check the wiring connection for corrosion inside.
Quote from: Remle on September 23, 2017, 08:38:58 AM
As you say swapping the wires should make the problem switch sides, but it does not. That leads me to ask, does this model have an ignition trigger? Seems I read that it has a two circuit system, one side could be bad ? Be careful testing that, as they say that they are easily destroyed. First check the wiring connection for corrosion inside.
One set of points for both sides Fires both plugs at the same time.No ignition trigger.
No flywheel magnets for spark.
Not much left to try.
Is the piston moving up and down? ;)
Fire, fuel and air. If it gets those in the proper amount and at the proper time, it'll run.
One or more of those is not arriving on time or in the right amount.
After it fires, it has to be able to fart. Clogged exhaust won't keep it from firing, but will stop it from running afterwards.
May-be you can give it a "hot supper" hook it to another engine by belt and
let it turn over while you tinker with her,this might give it some incentive to
fire up. Had a Wisconsin V-4 that we had to do this to and this made her come
coco just needed a little power boost. Keep trying it's just gotta go.
Oh No! Getting it to run would ruin a perfectly good topic! ::)
Does the coil look like this one?
(https://forestryforum.com/gallery/albums/userpics/19307/.jpg?easyrotate_cache=1403810613)
The new ones from Cummins/Onan look like this . . .
(https://forestryforum.com/gallery/albums/userpics/19307/Onan_coil_new__1.jpg?easyrotate_cache=1403810610)
Quote from: Andries on September 23, 2017, 12:15:16 PM
Does the coil look like this one?
(https://forestryforum.com/gallery/albums/userpics/19307/.jpg?easyrotate_cache=1403810613)
The new ones from Cummins/Onan look like this . . .
(https://forestryforum.com/gallery/albums/userpics/19307/Onan_coil_new__1.jpg?easyrotate_cache=1403810610)
That looks like a Kohler coil. My coils look like this.
(https://forestryforum.com/gallery/albums/userpics/39553/coil.jpg?easyrotate_cache=1506183768)
Quote from: Kbeitz on September 23, 2017, 12:23:03 PM
That looks like a Kohler coil. My coils look like this.
The box it came in said Onan/Cummins.
It was bought at the local Cummins dealer.
I don't own any Kohler motors.
You may have identified your motors problem . . . ?
Quote from: Magicman on September 23, 2017, 10:41:33 AM
Oh No! Getting it to run would ruin a perfectly good topic! ::)
True true you have a good point,but I gotta keep K interested he seems like he's going to stall all this giving up talk,all done,no way I say,no way.
Does that thing have an impulse carb like a Briggs ?If so they can give you fits if the diaphragms get funky .It's a float carb with a diaphragm for some reason .The parts are cheap if that be the case .If it's firing both plugs and only putts on one the only thing left is fuel delivery .
The impulse pump on my 24 hp Onan, looked like this:
(https://forestryforum.com/gallery/albums/userpics/19307/LT30G242C_fuel_pump_impulse_type.jpg?easyrotate_cache=1351268219)
I replaced it with this:
(https://forestryforum.com/gallery/albums/userpics/19307/LT30G242C_fuel_pump_electric_type.jpg?easyrotate_cache=1412572240)
Well I got it running... But I have no idea why.
First thing today I put a different set of points in it
out of one of my running tractors. It made it worse.
I put the point back in the donor tractor and it worked
fine. I was about to give up and I had another parts
engine with points. I looked it over and thought that
it might be easier to remove the whole point box instead.
Well the definitely was not easier . Then when I tried to
start the engine it was running again on one cly. But this
time the engine started smoking real bad. Then I noticed
that it was running on the front cylinder and not the back.
this is why all the smoke. From all the running on the back
the front loaded up with gas and oil. Then I seen the back plug
wire was not on all the way. I put the wire on and it fire right
up and ran fine... So... What did this problem have to do with
the point box?. I'm thinking maybe it had a bad ground.
But why did the spark look so good but would not run the front
cylinder but would run the back so good. I tried putting the other
coils back on and they all work great. Same with my spark
plugs. they all work good. So I'm really lost on why changing
the point box fixed my problem. This was the forth set of point
and condenser that I tried.
Non working points box.
(https://forestryforum.com/gallery/albums/userpics/39553/Points_box.JPG?easyrotate_cache=1506200684)
A ride the the mountain top.
(https://forestryforum.com/gallery/albums/userpics/39553/DSC06814.JPG?easyrotate_cache=1506200807)
Thanks for everyone's help.
Oh No, there goes the Neighborhood!!! :-\
Anyone have any idea what might have happened ?
I'm still stumped... Why did that fix it?
Hard to say why it works now, but you get points (get it?) for hanging in there. 8)
Quote from: Kbeitz on September 23, 2017, 07:02:08 PM
Anyone have any idea what might have happened ?
I'm still stumped... Why did that fix it?
Like you said faulty point box,what else could it be with all the work you did on it
it should run like a new one. I just knew you could do it but we all like a challenge
what fun would it be if it was simple?
HURRAH GOOD JOB!
But I like to learn from what I did.. I learned nothing...
A stupid little aluminum box should not control where
a spark goes. No logic... I'm not even sure it was a spark
problem because I seen a nice blue spark at the non running
plug at all times. Swapping the spark plug wire from one side
of the coil did nothing... I wont sleep tonight.
You have to get some sleep because you have to get up and do it all over again tomorrow. Plus the World did not End today . Life is Good.
Apparently the box was not (fully) grounded which means that the condenser was not either. Of course it should spark without the condenser, but the points would suffer. Clean the grounding surface and reinstall the old box.
EDIT: Well that doesn't make sense because the same points and condenser services both sides??
Correct me if I'm wrong but as I recall and it's been over 30 years the points set up on the top of the engine .The works are transmitted via shaft from the cam or crank,but I never got into one that deep .The one and only was a welder with an Onan but labeled as something else,Miller maybe .It was a good machine have to say that .Took me a while to figure out where the points were ,just a little tweek,they were not closing all the way.Glad it putts on two.
Funny thing... both engines the one I was fixing and the donor engine has
What looks like a factory gasket between the engine and the points box.
You would think that that would defeat the box ground.
Yes the points box is on top of the engine mounted under the carberator.
Quote from: Kbeitz on September 23, 2017, 07:02:08 PM
Anyone have any idea what might have happened ?
I'm still stumped... Why did that fix it?
Gremlins. Probably the same ones who slip into my kitchen and swap out my plastic storage container lids so that I have containers and lids that won't fit each other.
Quote from: Kbeitz on September 23, 2017, 08:09:11 PM
But I like to learn from what I did.. I learned nothing...
A stupid little aluminum box should not control where
a spark goes. No logic... I'm not even sure it was a spark
problem because I seen a nice blue spark at the non running
plug at all times. Swapping the spark plug wire from one side
of the coil did nothing... I wont sleep tonight.
I know you wont sleep I wouldn't ether till I really knew,but I also know that you
will get the answer sooner or later,just let us know.
This:
(https://forestryforum.com/gallery/albums/userpics/39553/Points_box.JPG?easyrotate_cache=1506200684)
looks suspicious.
Can you get a better crop of this?
(https://forestryforum.com/gallery/albums/userpics/38564/Points_box-edited.jpg?easyrotate_cache=1506221768)
My son once had a 1985 Caprice Classic. It was changed over from 4 bbl carb to electronic fuel injection. Every time it rained, the engine died. Turning left while accelerating, the engine died. No tests showed anything wrong. I finally fired it up in the garage and poured buckets of water over the windshield. Turned on the wipers and the engine died.
The multiconductor cable from the ECM to the engine compartment passed close enough by the wipers to get smacked by their actuator on every rotation. There was no visible damage to the outside of the cable, but the wires inside were broken. As the cable was lifted, those wires acted as open switches. As the cable dropped back down, they reconnected inside the insulation.
Quote from: grouch on September 23, 2017, 11:00:42 PM
This:
(https://forestryforum.com/gallery/albums/userpics/39553/Points_box.JPG?easyrotate_cache=1506200684)
looks suspicious.
Can you get a better crop of this?
(https://forestryforum.com/gallery/albums/userpics/38564/Points_box-edited.jpg?easyrotate_cache=1506221768)
Seen that and I had the wire pulled out from the box while testing...
Thanks.
PDF shows a gasket under the box...
(https://forestryforum.com/gallery/albums/userpics/39553/Coil_box_Onan.jpg?easyrotate_cache=1506222560)
A long time ago, my 1968 VW square back had the wire exiting the distributor rub a bare spot on the bottom. It would sometimes ground out, killing the engine. Took quite a while to find that one.
Correct me if I am wrong, but doesn't it just have one set of points? If that wire was grounded it would have killed both cylinders, or you would at least think so. I know I pinched the wire to a set of points under the dist cap once, it killed all 8 cylinders and that was without even a break in the insulation.
It is a head scratcher for sure, but logically it could not have been the points UNLESS the box could have been in a position that was not letting the points fire when they needed to to run that cylinder, which would also explain why you had blue spark, but a no fire in the hole situation.
If it fires both sides of the coil every revolution, it must have been just firing every other revolution and the blue spark you were observing was the wasted spark.. Something to think about maybe.
That would also explain why it would still run on the same cylinder when you swapped the plug wires.
Good thought... Best so far...
This drawing has me scratching my head...
(https://forestryforum.com/gallery/albums/userpics/39553/Cam_shaft.jpg?easyrotate_cache=1506250994)
You would think the points cam would have a double sided lobe.
(https://forestryforum.com/gallery/albums/userpics/39553/Camshaft_2.jpg?easyrotate_cache=1506251429)
Did you not say both fired at the same time,one lobe one firing for both
this would enplane why it's designed this way,no,yes,may-be?
On your cam shaft drawing, are you sure you have that right... Wouldn't there need to be two lobes per cylinder (two valves each) or are they exactly opposed and use the same lobe to control two valves?
That is correct on a horizontally opposed twin. Remember that the cam rotates at 1/2 crankshaft speed so one lobe operates the valves for both cylinders. The points open every revolution of the cam which doesn't matter because when one cylinder fires the other is near the top of the exhaust stroke. This is where the 'wasted spark' comes in, it's not producing any power
Quote from: sawguy21 on September 24, 2017, 11:57:29 AM
That is correct on a horizontally opposed twin. Remember that the cam rotates at 1/2 crankshaft speed so one lobe operates the valves for both cylinders. The points open every revolution of the cam which doesn't matter because when one cylinder fires the other is near the top of the exhaust stroke. This is where the 'wasted spark' comes in, it's not producing any power
So without a double lobe on the points, there would only be one spark per full cycle. On engines that I have worked on, the points were run from the crank shaft. If that were the case here, then there should be enough fires of the spark to hit both cylinders.
Quote from: ljohnsaw on September 24, 2017, 11:39:59 AM
On your cam shaft drawing, are you sure you have that right... Wouldn't there need to be two lobes per cylinder (two valves each) or are they exactly opposed and use the same lobe to control two valves?
exactly opposed
Today I jumped on the tractor to use it... It would not start...
I worked on if for around 1/2 hour... it was out of gas....
I don't think this tractor likes me...
Kbeitz, you can be a funny guy on a Monday morning!
So, after this 'Onan Odyssey', my take on the problem is that somehow the box holding the points and condenser wasn'T lined up with the plunger that opens the points. Everything else was good, but the "timing" was off ?
. . . or was it the gremlin at work and now he's draining gas from the tank? :D
Go back to bed, get up from the other side, have a fresh coffee and start over. :D My neighbor was having trouble with his Kubota, it sounded like a dead cylinder. After fighting with it and calling the service truck, he realized he had filled it with lawn mower gas. His face was red too.
Quote from: Andries on September 25, 2017, 10:28:53 AM
Kbeitz, you can be a funny guy on a Monday morning!
So, after this 'Onan Odyssey', my take on the problem is that somehow the box holding the points and condenser wasn'T lined up with the plunger that opens the points. Everything else was good, but the "timing" was off ?
. . . or was it the gremlin at work and now he's draining gas from the tank? :D
All just had to be gremlins... All of it...
Quote
exactly opposed
I've never had an Onan apart before. The most I've ever had to fix was a loose exhaust valve seat. How do they clear the rods when the cylinders are exactly opposite? Are they bent/offset a little down near the crank?
It's good it's running.
Is that ignition box supposed to be grounded? If so, maybe the bolts that grounded it were buggered up just enough to not let the other side fire? As in the other side had more resistance for whatever reason.
I also hate it when something is all of a sudden working again and I don't know why. Don't feel like the lone ranger.
The cylinders are offset on the block.
I guess there is a chance you could have unknowingly fixed the problem a while back, if there was enough gas and oil in the cylinder to keep the plug flooded and stop it from firing.
I have at times taken a plug wire off the plug on a dead cylinder and held it off the top of the plug enough (¼" ish) that the spark would jump to the plug (with insulated pliers), sometimes this would give the spark enough boost to get it going.
Quote from: Hilltop366 on September 26, 2017, 11:35:11 AM
I guess there is a chance you could have unknowingly fixed the problem a while back, if there was enough gas and oil in the cylinder to keep the plug flooded and stop it from firing.
I have at times taken a plug wire off the plug on a dead cylinder and held it off the top of the plug enough (¼" ish) that the spark would jump to the plug (with insulated pliers), sometimes this would give the spark enough boost to get it going.
I had the head off a few times... New plugs many times...