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Walhonding520

“Puff” now won’t run

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Walhonding520

520h with Onan has been running great for last 2 years. Mowed tonight for about 30 minutes. Jumped off with engine running a bit over idle speed and walked a few feet away. Heard a “puff” and kind of “bang” sound at the same time. Instantly started running rough and vibrating hard. Let it run for a minute than shut it off. It would start the first 2 times I tried, but only with choke on and wouldn’t keep running. Won’t run at all now but turns over fine.

At a loss for ideas. Did I blow a gasket?

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Achto

Pull the spark plugs out and check the compression on each cylinder. No need for a gauge or any thing, just hold your finger over the plug hole while you turn the engine over. There is a possibility that you lost compression on one cylinder.:(

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Walhonding520

What would most likely cause loss of compression that would make the sound I described?

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Darb1964

Did you check to see if you are getting a good supply of fuel. I left my C81 out in a two inch rain storm few days ago. I was working with it, it's my bucket tractor and it started running poorly and lost power and stalled. Water in the fuel, I also noticed the fuel filter was in need of replacement.

Something to check before going to far.

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Walhonding520

I pulled the plugs. The front cylinder about sucked my finger in but very little on the rear cylinder. 

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Crazyredhorse
Posted (edited)

check for spark,check plugs,if ok put a little gas in each cyl.try that if it fires it's got to do with gas.if not could be a valve.if you can get it running pull plug wire on each cyl.while running.if I dosen't change the way it runs you know which side is bad.but if nothing's clanking or banging while your turning it over doesn't sound like rods or pistons.or something like that.look around in cylinders with flashlight too if ya can.

not to scare you but just so you know.rear cylinder is the problem too. possibly jumped timing or something like that to.dont know about your motor myself.

Edited by Crazyredhorse
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lynnmor
5 hours ago, Achto said:

Pull the spark plugs out and check the compression on each cylinder. There is a possibility that you lost compression on one cylinder.:(

 

Report back with the compression readings.

 

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Achto
6 hours ago, Walhonding520 said:

I pulled the plugs. The front cylinder about sucked my finger in but very little on the rear cylinder. 

 

When you turn it over the compression should try to blow you finger off from the hole. If the rear cylinder had little to no compression you could have a broken rings, hole in the piston, broken rod, stuck valve, etc. Sadly no compression usually requires opening the engine up to see what happened.:(

 

Before tearing the engine down, I would do a compression test with a gauge. Better yet, a leak down test will tell you where you are loosing compression.

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WHX??
9 hours ago, Walhonding520 said:

What would most likely cause loss of compression that would make the sound I described?

The dreaded valve seat...:o

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Walhonding520
Posted (edited)

Im going to run into town and rent a leakdown kit.  A guy on another site suggested the head gasket because I heard a sound like air puffing out.

 

The rear plug was a little loose.  I could wiggle it just a little before I pulled it but it doesn't seem to me that it would cause a lack of compression without the plug in.

 

If it the valve seat, what kind of repair am I looking at.  I understand I will have to tear off the top of the engine but I don't have any experience working with or adjusting valves or working on valves in general.  Would it be a good idea to find a small engine guy and just have them do it?

Edited by Walhonding520
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lynnmor
59 minutes ago, Walhonding520 said:

If it the valve seat, what kind of repair am I looking at.  I understand I will have to tear off the top of the engine but I don't have any experience working with or adjusting valves or working on valves in general.  Would it be a good idea to find a small engine guy and just have them do it?

 

Until we have the compression test, all of this is just a guess.  If a valve seat popped out it the repair can range from fairly simple up to an engine replacement.  We will all know more when you report a low compression and then pop off the head.

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Bill D

Bang equals rod failure in my opinion.  Just a guess.

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Rob R
1 hour ago, Bill D said:

Bang equals rod failure in my opinion.  Just a guess.

With you on this one Bill el rod-o probably broken-o..........

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Walhonding520

When I turn the belt pulley the valves seem to close up fine in both cylinders. The head gasket wasn’t perfect but not bad. The piston doesn’t move when I turn the pulley. Does this mean broken rod?

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lynnmor
2 hours ago, Walhonding520 said:

When I turn the belt pulley the valves seem to close up fine in both cylinders. The head gasket wasn’t perfect but not bad. The piston doesn’t move when I turn the pulley. Does this mean broken rod?

 

Yep!

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Crazyredhorse

Ouch:text-imsorry:

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Gregor
2 hours ago, Walhonding520 said:

 Does this mean broken rod?

Broken rod does not necessarily mean destroyed engine, but it could get spendy. :scared-shocked:

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WHX??

Dang it ... well better then the dreaded valve seat anyway. 

Let not :scared-eek: the guy yet Gregor..... might be able to just replace the rod? 

But motor does have to get torn down. 

Hoping for the best here Wal. Least block did not get ventilated. 

What's the thoughts on just matching up a new rod with the existing gizzards? Big end clearances checked of course. 

Bigger ? Is why did the rod go? Only a tear down wil answer that. 

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Walhonding520

Ok so next then is to start tearing it apart I guess.  I think if I took the time to do it, I could.  Problem is I don't have a lot of time and its my only mowing tractor.  I may try to find a local guy that likes working on small engines like this and see what he says.

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WHX??

Good luck with the local guy Wal .... most around here anyhoo  are overworked, under loved and swamped not to mention not even wanting to touch an Onan due to expenses that @Gregor mentioned. Keep us posted tho. 

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Walhonding520
Posted (edited)

What causes a rod to go.  I do good maintenance and the oil level was fine.  It is an older engine with a lot of hours on it. 

 

Might it be a better idea to swap a new engine on?  What would be a good option for that?

 

I did mention before that the front cylinder had a lot of compression (119).  Could this be a good sign that there isn't to much damage internally?  I did push the piston in as far as i could and the cylinder seems ok and the piston moved smoothly.  Maybe just have to clean things up well and replace the rod?

Edited by Walhonding520

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Achto
1 hour ago, Walhonding520 said:

Might it be a better idea to swap a new engine on?  What would be a good option for that?

 

 

Some guys have installed a Harbor Freight Predator 22 in their 520's

 

1 hour ago, Walhonding520 said:

I did mention before that the front cylinder had a lot of compression (119).  Could this be a good sign that there isn't to much damage internally?

 

Unfortunately this question is not able to be answered until you open the engine up. The rear rod may have broken and caused little to no damage, or it could have torn things all to crap inside.  

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Tuneup

The bang was the big indicator. I'd locate a cheap MTD POS and mow with it for the season while taking that Onan apart. Lots of talk about the rear cylinder running hotter than the front, causing a bad seat but a rod? Unknown. You'll want to get that case opened ASAP to see if it's salvageable. I'd lend you my old Snapper but you're a little too far away...

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Walhonding520

I may be able to find a loaner until it i repaired.  

 

I don't think GA is to far for a good old snapper Yuneup :D

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tom2p
Posted (edited)
12 hours ago, Walhonding520 said:

What causes a rod to go.  I do good maintenance and the oil level was fine.  It is an older engine with a lot of hours on it. 

 


how many hours is a 'lot of hours' ?

 

were the valves adjusted or decarbon done ?

 

run a lot with debris clogging the air intake and / or cylinder and head fins ?


if motor oil was maintained (changed and correct level) it is possible it was (basically) running on one cylinder (for some time) and this could have led to a broken rod 



if the tractor is in good shape otherwise including trans and mower deck - might be worth the time and money to rebuild the engine ... and especially if you require the power of a 520 or similar 


otherwise - might be better to look for another tractor (Wheel Horse of course)

 

Edited by tom2p

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