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zooms

running a 520 with low compression in one cylinder?

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zooms

Hey guys, I have a thoroughly abused 520 that a buddy gave me. I've been slowly checking it out and making a list of things that need to be fixed. I just pulled the spark plugs and sprayed a little oil down the cylinders and spun the starter to check for compression (without a gauge). The front cylinder has great compression (almost sucked my finger in), the back one has almost nothing. Will I damage anything if I get it running with the bad cylinder to continue checking out what needs to be fixed? In other words would it OK to run a 520 with bad compression on a cylinder? I won't be working it or anything. Does anyone know how involved it is to replace the rings on the back piston?

 

For those who may care, here is my list thus far:

[ ] Check valve clearance
[ ] Replace spark plugs
[ ] Replace air filter
[ ] Replace fuel filters
[ ] Change oil
[ ] Find air box cover
[ ] Replace? Fuel pump
[ ] Replace fuel lines
[ ] Intake, exhaust and carb gaskets
[ ] Clean carb
[ ] Fix choke
[ ] Replace burnt wires
[ ] Replace fuel cutoff
[ ] Fix compression in back cylinder
[ ] Replace rear lights

 

If there's anything else I should be checking out, please let me know. This is my first horse, my first riding mower, and my first project. So I don't exactly know what I'm doing. And like I said, this was abused. You'd never guess it has 300 something hours on it. Thanks so much.

 

 

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lynnmor

If you just want to get it started it might be OK, but don't continue to run it with low compression.  If it is the dreaded valve seat, it might be repairable now but at some point it will be too far gone.

 

I had 75 lbs on a front cylinder and it was a simple valve grind to bring it back up to 125 lbs.

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zooms

Thanks Lynnmor. So you're saying it may be the valves leaking instead of the piston rings? I'm mechanical/engineering inclined, I just have no experience with engines of any kind. How does one grind the valve seats? I would imagine making sure the grind is even and true is critical. Is that the sort of thing you have to send out for?

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oldredrider

Valve seat grinding is done by an engine machine shop. The problem with the Onan engines is the valve seats can come loose from over heating the engine, poor maintenance etc. You said the tractor was abused. The "compression test" you performed is indicative of poor maintenance and/or abuse.

At the least, if that's what's wrong, the valve seats will need to be repaired or replaced.

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zooms

Thanks oldredrider. So the valve seats can come loose? I thought the valve seats and the head were just cast as one piece.

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lynnmor
11 hours ago, zooms said:

Thanks oldredrider. So the valve seats can come loose? I thought the valve seats and the head were just cast as one piece.

The seats are steel rings pressed into the aluminum block.  The different amounts of thermal expansion can cause them to loosen.  Once loose, the block wears away and may become damaged beyond repair.  That's why you don't want to continue operation.  This valve seat problem seems to be the worst problem with the otherwise good Onan engines. 

 

There are .010" oversize seats available and there is a company that makes seats that are much larger.  Either will require machining to install.  Some have even welded the block in severe cases.  Some automotive machine shops are good at doing the repair, others not so much.

 

If you are very, very lucky, a simple valve grind will be your fix.

 

To prevent future valve seat failures, you should insure that the oil filter mount is not weeping oil and causing dirt buildup.  Keep the cylinder fins clean.  There should be a rubber grommet, (often missing) around the oil filter to prevent cooling air loss there.

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zooms

thanks again lynnmore, you guys really know your stuff. I was talking to my boss this morning about the issue and he thinks it's just a stuck valve (this tractor sat for a year or two unused) because I have basically no compression and if it was leaking around the valve seat he thinks there would be some compression. I really hope that's right, because it's a lot easier to fix. Does that sound right to you guys? He knows a ton about fixing engines in general, but you guys know these particular engines really well. It might be a while before I have a chance to pull the head off and see if a valve is stuck. Thanks again for all your help.

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lynnmor

You can check for a stuck valve by taking off the valve cover.  It will require removing the exhaust heat shields, exhaust and intake systems.  It needs to be done anyway so that valves can be adjusted, ground, replaced or whatever repairs are needed.  Rotate the engine by hand and watch the valve movement to check for sticking.

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zooms

lucky for me I already have all that ripped off. I was going to see if it would run when I squirt gas into the spark plug holes, before moving on to cleaning the carburetor and testing it without then with the fuel pump. Since it's electric start, what's the best way to spin the motor by hand?

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lynnmor

Just grab the inner pulley by hand and turn it counter-clockwise.  To make it real easy, remove the spark plugs.

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zooms

Wow....

 

[Insert face-palm emoji]

 

now that you mention it, that seems incredibly obvious:

 

Turn the big pulley attached to the engine.

 

Thanks again.

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zooms

Thanks again guys for all your help. I was able to check out the engine the other day. The vales move freely and the rings are sealing because the port to the back side of the pistons is blowing and sucking when I turn the engine. I even backed off the things (lifters?) that push the valves open to make sure it was able to close all the way. still low compression. At this point, I guess I'll have to unbolt the engine and rotate it so I can get the head off and take a look. It still could be just some gunk on the vales, but at this point I'm starting to prepare myself for a loose valve seat :no::crying-yellow:. I know prices vary wildly from place to place, but is there anyone out there that has had this done? Is it a ~$100 fix, a ~$500 fix, a ~$1000 fix? Is it the sort of thing that you just get a new engine for? Thanks for any input you guys have.

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woodchuckfarmer

I had 2 done at an engine shop, cost was $135.          Wayne

 

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Duramax7man7

Sorry for resurrecting an old thread but what was the verdict on the cylinder with low compression? 

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zooms

Wow, thanks for the interest. Life has gotten crazy busy and so taking the head off and taking a look at the valve seats has been pushed to the back burner, especially since my father in law is giving me his riding mower. At this point I'm just assuming it is loose valve seats. I may tear into it this summer to verify, I'm just not sure if it's worth the effort (it needs a ton done to make it just operational, and it was utterly abused).

 

Are you having the same problem?

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Duramax7man7

Yes, I purchase one for a great price and it was left in the weather to rot. But I saved it long before that. Mine wouldn't even power up. I had to clean the ignition switch well and wires. Replaced the wiring hardness Molex with bullet style connectors like on my first to create a good connection again and then trace out the wiring to a faulty switch relay. Starts now but I went through basically everything possible and that rear cylinder is still not firing. Even found the condenser wire connected to the positive terminal on the coil as well as my secondary coil windings have a smidge to much resistance coming in at 20,510ohm.  

 I'm going to be pulling the engine this weekend and decarbonize it and hopefully will only fine heavy carbon buildup or a stuck valve being set out in the weather like it was. I will do what I need to though. Read on another forum last night before seeing your thread on google and this was all that was wrong with theirs was carbon and needed a good cleaning and they got theirs running at full power again. They did perform compression tests though and were getting well above 100 on both cylinders. That will be all I'm asking for at this point. So hopefully I'll find a badly out of adjust valve and carbon. That will cause a dead cylinder. I have spark in both cylinders and front cylinder runs fine when I pull the plug wire on the rear one.

 I don't think that this will be a case of loose valve seat at this point as my 1990 520h that runs great has over 2300 hours on it and still runs strong, other than the oil seal leak that I'm dealing with after replacing the rear bearing plate gasket last weekend. ha ha. But this 1991 520h has only a little over 1200hours on it and there was nowhere near the debris in the shrouds that my 1990 had. No oil leak residue or anything. Heck for all I know there could be part of an acorn in one of the valve ports! ha ha. With all of the acorn shells that shot out of the intake tube on top of the shroud when I got it to start for the first time. 

 Here is the thread on it.

 

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zooms

I hope your valve seats are ok. My 520 has 320 hours on it. 320 hours was all it took for someone to totally abuse it to the point that I may not fix it. I was thinking that I would use it as a small tractor and get a tiller for it, but I live in central Arkansas and their aren't very many wheel horse owners down here.

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Duramax7man7

Thanks! Me too! I do remember that when I took my 1990 520h apart, it was pretty dirty inside. I put it through a de-carbon process, reset the float and air mixture screw as well as got new wires for it which was the main culprit of the rough running in the end and then had to get a new stator and went ahead and got a new VR as well and removed the Molex from the wiring harness, replaced the flywheel and pinion gear as well as just replaced the plastic governor ball spacer with a metal one and she runs great now overall. It had 2300+ hours on it. Had TONS of sludge buildup under the shroud and was packed with it around the stator. 

 SO being this 1991 only has 1200+ hours on it and is actually very clean overall with only some cotton loosely collected around in the shroud and some sticks and acorn shells here and there, I'm hoping it's something simply at this point. I'll be tearing into the engine tonight. 

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Searcher60

What year is your 520 with 320 hrs. on it? Is it for sale? 

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zooms

It is a 91. It's in pretty rough shape. And, probably more importantly, it's in central arkansas.

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