Anyone have any experience with twin cylinder Onan engines?

[steved]

I have a 16HP twin cylinder Onan that misses sporadically, and only on one cylinder...when it runs right, its runs good.

I thought bad gas initially, but it should miss on both cylinders if that was the case. Adding fresh gas did seem to help, but when I started it tonight it missed like it did before. It backfired badly tonight, which it didn't do earlier.

We replaced the plug wires and the plugs...no change.

I'm thinking its either a coil issue (not sure why it would fire one side and not the other), or possibly a compression issue. I'm not sure what the ignition system on a Onan looks like...could be a condensor issue.

This is for my old beasty Gravely garden tractor, soon to be firewood hauler.

Thoughts?

 

[nathon918]

with the backfiring it sounds more like an intake valve not seating properly...

 

[albert]

Sticking valve or too tight adjustment. If it has or had bad gas in it try fresh gas with a cleaner, seafoam or such. It may help free a sticking valve. Compare spark when it misses to rule out ignition.

 

[dieselfitter]

I've seen the screws holding the air cleaner to the carburator loosen up and get sucked into the engine and hang up the inlet valves.

 

[vinced]

Twin Onans fire both cylinders at the same time. I'd put in a new set of points. If that doesn't do it then look at the valves. A compression check will tell a lot.

 

[Ed*L]

The first generation Onan twins had issues, Deere used them for a while, made a lot of money selling replacement engines.

Valve seats coming loose in the heads were a common issue, had a 20hp that did that. My 16hp had the cam bearing spin in the end plate, took a while to figure out why the points wouldn't stay adjusted for more than 3 seconds.

Ed

 

[goosegunner]

My 1985 John Deere 318 has acted that way a few different times. Low power under load and missing. Both times a change of condenser and points fixed the problem.

gg

 

[John R]

Those engines are famous for the valve seats coming loose, and pulling out of the block.

 

[steved]

I did some more research and thinking on this thing, here are some things that I came up with.

I'll have to check for points and condensor as gg eluded to...I almost want to say this one has an (very pricey) electronic pickup on this model? One of these (many) tractors had a pickup on it in place of points...I'm pretty sure its this one.

I don't think its valves, all small engines I've dealt with that had valve issues got worse as they got hot...there is no connection between my missing cylinder and how hot the engine is. One minute it purrs like a kitten, the next its running on a single cylinder like somebody flipped a switch. I have to pull the carb to check the valve lash from what I've read; so maybe pull the carb for a cleaning and check the lash.

I think I narrowed the gas out...why would gas affect one cylinder and not the other? Its definitely only one cylinder (left bank, sitting on the seat). Gasoline was treated, albeit about six months ago.

As for the backfire, here's a possibility that I thought about after learning how the coil fires. 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...the coil could simply be firing during the exhaust stroke (cylinder full of raw gas and exhaust valve open)?? The coil is relatively easy to diagnose...switch plug wires from terminal to terminal, see if the other cylinder goes dead and the dead one comes to life? This is a true coil (not a magneto), with two plug wire towers...its also original (circa 1970) as it is painted factory Gravely yellow. The coil is around $100.

Everything I read on the interweb indicated a failing coil (if its on the webs, its got to be true...). I've seen twin tower coils used before in Kohler twins, and when they failed, they died; none of this running on one cylinder BS. Never had a coil that would partially fail.

Thanks for all the input...I think my next (easy) course of action will be to swap the high test leads on the coil, see what that does. If that doesn't change anything, then I would agree its likely a valve/compression issue.

 

[nathon918]

As for the backfire, here's a possibility that I thought about after learning how the coil fires. 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...the coil could simply be firing during the exhaust stroke (cylinder full of raw gas and exhaust valve open)??

like I said, the backfiring indicates more of a valve problem, as you couldn't backfire with no spark...
id think its more of a sticking valve, after running for a minute the valve stem has probably expanded enough to stick in the guide, then when it cools it shrinks and closes...
with the ignition system it has it makes me lean towards a valve problem, but I too would go ahead and swap the plug wires to rule out the coil, then if its fine, check compression

 

[steved]

Where I was going with that was an intermittent spark, no spark on the compeession stroke and spark on the exhaust stroke...its not all the time and only happened after the machine was flooded from the 300 mile ride home in the bed of my 2500.

I'm not ruling out mechanical issues, but not going with worst case scenario quite yet. ..

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[John R]

like I said, the backfiring indicates more of a valve problem, as you couldn't backfire with no spark...
id think its more of a sticking valve, after running for a minute the valve stem has probably expanded enough to stick in the guide, then when it cools it shrinks and closes...
with the ignition system it has it makes me lean towards a valve problem, but I too would go ahead and swap the plug wires to rule out the coil, then if its fine, check compression

The coil only has one set of windings in it, it either works on both towers , or it doesn't work at all.
Now maybe a bad plug wire, but my money is with the valves.

Keep us posted on what you find out.

 

[hanniedog]

Saw a Cub Cadet 1912 lawn tractor on C-list made in 85-86 with a Onan 20hp engine. Is this one to shy away from?

 

[UpOnTheHill]

My vote is valve seats also. Onan twins are famous for the seats needing work. I had a p218 in my JD 318 which needed new seats. It had some blow by. I kept fresh oil in it and ran it many hours like that. It had a little over 2600 hours on the meter before I sold it about a year ago. As far as I know, it never had a rebuild.

 

[steved]

The coil only has one set of windings in it, it either works on both towers , or it doesn't work at all.
Now maybe a bad plug wire, but my money is with the valves.

Keep us posted on what you find out.

Actually, from what I have been told by some Gravely guys, there are two sets of windings in this particular Onan coil...it can fail halfway and cause the type of pull-out-your-hair fits I'm seeing. I found that out last night...news to me, but if true; that could explain a lot. Keep in mind, this coil is nearly 45 years old at a minimum. So there is no shame in it if it has failed...

I went out last night and like a fool, swapped the plug wires from tower to tower before I tried starting it...it ran absolutely fine, for the most part. Now, it was under 40*F when I fought it the other day and it was nearly 70*F last night. I drove it around some to get it hot, I wanted to change oil. I found that it tends to have a single miss (pop) at mid to low idle, and runs perfect at full throttle. I have no BACKfiring, and I want to think that was from being flooded from the long bumpy ride home.

I changed the oil and filter, it needed it (did I mention this is a new-to-me tractor). Probably order a coil for it next week, just so I can throw a part at it.

 

[GVS]

I have a 16HP twin cylinder Onan that misses sporadically, and only on one cylinder...when it runs right, its runs good.

I thought bad gas initially, but it should miss on both cylinders if that was the case. Adding fresh gas did seem to help, but when I started it tonight it missed like it did before. It backfired badly tonight, which it didn't do earlier.

We replaced the plug wires and the plugs...no change.

I'm thinking its either a coil issue (not sure why it would fire one side and not the other), or possibly a compression issue. I'm not sure what the ignition system on a Onan looks like...could be a condensor issue.

This is for my old beasty Gravely garden tractor, soon to be firewood hauler.

Thoughts?

I'd do a hot copression check first to determine a no valve issue then check the ignition.

 

[Sawyer Rob]

I have one of these motors complete, that I don't need, it's either 16 or 18hp.

If anyone is interested just PM me...

SR

 

[steved]

I have one of these motors complete, that I don't need, it's either 16 or 18hp.

If anyone is interested just PM me...

SR

If this one goes south, it'll have a twin cylinder Kohler installed...from what I've been told; Onan's are expensive to rebuild.

 

[XSKIER]

The intake manifold can leak and lean out one cylinder.

 

[steved]

Well, here is what fixed it...

Bought a new coil...same miss. Kinda disappointing. Remember someone said swap plugs and avoid Champions. So I figured I'd try it since that's cheap. Put two new Autolites in and not a miss in over two hours of run time. $5 in plugs would have saved me nearly $100...

Now to get the charging system fixed...

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