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Ponder

20 horse onan smoking

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Ponder

Hi guys. I am seriously interested in a very nice condition 520. It has the emblems in gold....anniversary edition? Anyway, I'm pretty sure the problem is the notorious valve seat issue. My question is, what can I expect to pay to have the engine rebuilt or valve seats done? It does run. Just smoking like a freight train.

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Lagersolut

Take this for what it's worth, my machine shop guy told me the Onan he did before my Kohler,  the guy had close to 2000 in his motor ....that was a full rebuild .

 

The valve seats I can't answer, one things for sure, the Onan repairs are pricey .

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Ponder

I know where I can buy a new onan 20hp still in the crate for $1600 but I do not want to drop that much into a mower. I found a used one that runs well for $850 but I know I could be on borrowed time with it. It has 560 hours on it. From what I have been told, if the current motor only needs the head done and bored .010 over with new Pistons and rings, I was told $700. But, that was just a guess on the part of the small engine shop mechanic.

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boomers_influence

ponder

a compression and leak down test will tell

where the problem is.

bad valve seats will usually have VERY LOW compression.

thank you. boomer

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Ponder

Ok guys I bought this 520. Let me just say this... immaculate condition! Ok. It does not smoke idling and only puts out a puff when I throttle it up. Air filter was really really dirty. I cleaned that and noticed the rubber line coming up into the bottom of the breather housing. It looks to me like it should have a pcv valve on it. It produces smoke from it also when it idles up. This has produced oil residue on the engine which I have pressure washed off. It has a slight miss too. It's not a bad miss by any means. But comparing it to my 416, you can tell it does have the slightest of a mis fire. With the mower running and in neutral, I checked the hydro fluid. It's not even touching the stick! It does seem to pull normal in forward and reverse. Am I not checking it correctly? You guys who have hydros give me some pointers please. Does any of your 520s have a pcv valve? I'm thinking this may be the entire trouble and the motor is fine. But, I could be wishful thinking.

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boovuc

Check the hydro rear end with the engine off Ponder.

The crankcase breather line you see does have a little valve and a filter pack in it at the motor's top. (The other end of the hose). Take it apart and clean the packing per the manual. (Download the manual from the manual section if you haven't done so already.

If you don't have a compression tester, buy a cheap one at Harbor freight and check the compression front and rear both cold and hot. 

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decksetter

That "slight miss" is probably the valves working themselves tight. If you don't know when the valves were adjusted, get them done ASAP.

I've worked on 5 onans and every single one of them worked its valves so tight the intake valve was held open enough to lose compression. If the valve isn't holding the seat in when it goes through the thermal cycles it will eventually pop the seat loose.

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snowmoboyle

I have a 89 520H - the miss you talk about, so thats the valves?  Mine runs smooth under load - snowblowerin' this winter it was strong.  But it would flutter here and there at idle or just driving around. Haven't had a chance to try it again since a carb clean, and finding out my keyswitch is bad causing spark issues!?!!?  

 

How long does it take to adjust valves - hour's labor at a shop? Seems like something I or my dad would be better off leaving our chubby fingers out of haha

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Ponder

Is it smoking? Have you ever had the valves adjusted? How many hours are on it? If it isn't smoking you are probably fine. You MUST be diciplined to always keep the fins on the rear cylinder clean. Not doing so will result in a costly rebuild like what I'm going through now. Although, you probably are due to have the head de-carboned and a valve adjustment. The engine will last forever if you take care of it. Keep the cylinder fins clean and after you get the valves adjusted and de-carboned, put 1 oz of seafoam brand gas additive per gallon of fuel, keep the air filter clean, change the oil and filter at least once every mowing season and it will last forever.

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snowmoboyle

It is not smoking - it has under 800 hours on it.  

 

Any ballpark of price on the head work and valve adjustment? 

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Ponder

The master wheelhorse certified tech I use said $400 for top end. 800- 1000 for total rebuild.

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Ponder

Mine only has 520 hours. The problem lies in the fact the previous owner did not know to clean the cooling fins on the back cylinder. That can destroy one in no time flat. That's why I said you MUST clean that back cylinder religiously if you want he onan engine to last. It's an excellent engine but suffers from a poor design as far as cooling. The back cylinder catches a lot of grass and dirt that you can't see unless you get under it or remove the engine tin.

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decksetter

No idea on price. I can set the valves and have it back together in an hour or two, but I've done it 5 or 6 times the last couple of years and I don't pull the heads. I do it all in frame.

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snowmoboyle

No idea on price. I can set the valves and have it back together in an hour or two, but I've done it 5 or 6 times the last couple of years and I don't pull the heads. I do it all in frame.

 

I see you are only a couple of states over, are you free tomorrow LOL

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decksetter

I see you are only a couple of states over, are you free tomorrow LOL

Actually tomorrow is my anniversary... I don't think my wife would like that!

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snowmoboyle

Wellllllllll Happy Anniversary!!

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Ponder

Happy Anniversary!

The wheel horse is now at the shop. The owner-mechanic said even though the compression on both cylinders are 95 pounds, there is more to it. He said he is pretty sure it will need rings and possibly valve guides. He did say it would not need a rebuild. He said it is too quite and smooth and too few hours to need a total rebuild. But, he also said there is no certainty until he opens it up.

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RMCIII

Ponder -

 

If there is (1) paragraph of advice I could provide, here it is. Make sure you check over the shops work. There have been several members in the past that have been very surprised to find out what the shop did not do, that they said they would do as part of the engine repair. You may know the shop very well and the mechanic. But they even get side tracked and sometimes, forget what they were doing, thus leaving some "nuts" and "bolts"undone. I would also ask to have your parts placed into a box for you to keep. < Never know what/where/when/how, but the bottom line is making sure the parts he provides, are Onan parts and not just something lying around the shop..

 

Not trying to put fear into you regarding the job the mechanic will do, it's merely called covering your assets.

 

Rob

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