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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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grouch

This:



looks suspicious.

Can you get a better crop of this?



Find something to do that interests you.

grouch

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.


Find something to do that interests you.

Kbeitz

Quote from: grouch on September 23, 2017, 11:00:42 PM
This:



looks suspicious.

Can you get a better crop of this?



Seen that and I had the wire pulled out from the box while testing...
Thanks.
Collector and builder of many things.
Love machine shop work
and Wood work shop work
And now a saw mill work

Kbeitz

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

Ljohnsaw

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.
John Sawicky

Just North-East of Sacramento...

SkyTrak 9038, Ford 545D FEL, Davis Little Monster backhoe, Case 16+4 Trencher, Home Built 42" capacity/36" cut Bandmill up to 54' long - using it all to build a timber frame cabin.

starmac

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.
Old LT40HD, old log truck, old MM forklift, and several huskies.

starmac

That would also explain why it would still run on the same cylinder when you swapped the plug wires.
Old LT40HD, old log truck, old MM forklift, and several huskies.

Kbeitz

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

Kbeitz

This drawing has me scratching my head...



 

You would think the points cam would have a double sided lobe.



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

whitepine2

 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?

Ljohnsaw

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?
John Sawicky

Just North-East of Sacramento...

SkyTrak 9038, Ford 545D FEL, Davis Little Monster backhoe, Case 16+4 Trencher, Home Built 42" capacity/36" cut Bandmill up to 54' long - using it all to build a timber frame cabin.

sawguy21

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
old age and treachery will always overcome youth and enthusiasm

Ljohnsaw

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.
John Sawicky

Just North-East of Sacramento...

SkyTrak 9038, Ford 545D FEL, Davis Little Monster backhoe, Case 16+4 Trencher, Home Built 42" capacity/36" cut Bandmill up to 54' long - using it all to build a timber frame cabin.

Kbeitz

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

Kbeitz

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

Andries

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
LT40G25
Ford 545D loader
Stihl chainsaws

sawguy21

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.
old age and treachery will always overcome youth and enthusiasm

Kbeitz

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

Ox

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.
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

sawguy21

old age and treachery will always overcome youth and enthusiasm

Hilltop366

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.

Kbeitz

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

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