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boovuc

Opened The Onan

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boovuc

Finally had some time while it did nothing but snow today to open up the Onan that was on my 520-H Loader tractor. This is a continuation from when I swapped out a spare P220 because of only 30PSI of rear cyl compression on the original engine.

Long story short......the first owner of the tractor had the original motor's rear cyl intake valve seat repaired not once but twice to where it is now .030 over. (You will see that easily from the pics of the front to rear cylinder). 

I didn't even put 500 hours on this motor as the third owner and it is really carboned up but the valve seat and valves look pretty good. It's using some oil but from what I could tell, the valve isn't the main culprit in the loss of compression. I'll know more when I get to the tappets. Looking around the heads and the gasket, it doesn't look like it was leaking around the head. I'll know more later this week when I get the intake off of it and adjust the valves. Again, it was in real need of a decarbon job! First pic is the front cyl. Next two are the rear cylinder with the close up of the over-sized valve.

 

 

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decksetter

Well that's... Interesting.

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AMC RULES

I've read you can fog a can of Seafoam into carb while it's running...

and eliminate that carbon buildup.  Any truth to that suggestion?    :scratchead:

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JackC

That carbon is ugly.  The intake valve does not appear to be seating that squarely in the larger valve seat. Maybe it is the camera angle?

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decksetter

I've read you can fog a can of Seafoam into carb while it's running...

and eliminate that carbon buildup. Any truth to that suggestion? :scratchead:

I've heard that too. I did that in the throttle body of a Dakota I had and it completely cured a miss that persisted after I changed plugs and wires.

I always run Seafoam in my tractor / power equipment gas (supposed to act as a stabilizer too) and once in a while I shoot some down the carb at about half throttle with a spray gun I got at harbor freight.

My 520 had about 700 hours when I got it and is around 770 now if i remember right, and I don't know if it's ever been decarboned. I don't plan on ever doing it unless a problem presents itself. I set the valves without pulling the engine or taking the heads off. I figure Seafoam is pretty good stuff and I'm going to see what happens, maybe I'll learn something along the way.

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boovuc

It is interesting!  :)

The valves are seating properly. I think it's the angle and shadowing your seeing. I see it too now but only on the picture.

 

I put Seafoam to this motor 4 or 5 times when the carb acted up. It doesn't remove hard carbon deposits though once cleaned out, it may help keep it cleaner....longer.

There is some carbon on that over-sized valve seat that may have some bearing on the compression loss. I can't wait to see how out of adjustment the valves are. My 416-8 Speed didn't even remotely look like these nor did the spare P220G I put in the loader. Oh well!  :angry-cussingblack:

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WH nut

That valve seat is shot, there is no reason for that valve to be sunk into the seat that far. The seat needs to be replaced.

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boovuc

Thanks Nut. I looked at it and compared to the other seats and to my untrained eye, it looked like a fair seal/seat.

I re-read the invoices that I received with the tractor from the second owner. (Wish I would have read all this stuff I was handed at the time of sale before I bought it)!!

 

WH Nut.......If what your saying is true and that seat is beaten into the block, then this block is probably toast and here's why:

 

This is a 1991 520-H. Reading from the invoices of the original owner............In October of 2005, the dealership states that, "compression low on rear cyl, (45PSI), and front is OK, (120 PSI). Found intake seat dislodged from the cyl block. Sent block, valves and head out to machine shop, machined seat area to oversize and install new seat, perform valve job and resurface head", etc etc. Total hours on the engine at this time is 865 hours.

 

Less than two years later, February 1st 2007, "tractor lacking power. performed compression test on rear cyl, (30 PSI), front OK (100 PSI)."

"Found valve seat beaten down into the block and cracked block around the seat. Took block to machine shop for evaluation. Machine shop installed a .060 oversize intake seat and counter-bored  a little smaller to get better press fit. Lapped all four valves and seats, set valve tappet clearance, cleaned cyl heads," etc etc. Total hours on the engine at this time is 935 hours. In 70 hours of use over a 17 month period, it took the seat out again! Wow! And now the seat is a .060 over!

 

.060!!!!!!!!!!!!! :scared-ghostface:  

 

I don't know. I don't think it can be repaired again. (or should be). Even the dealership at the very bottom of the invoice stated that "it will be the last time that seat can be repaired". The tractor now stands at just under 1200 hours so lets say this third valve seat failure was at <1199 hours. This means the .060 seat held up for around 260 hours. That is unacceptable.

 

I wonder if I should just buy a used standard block plus valve, and transfer my parts over. I don't think I put more than 100 hours on that motor during my ownership.  What ever the case may be, this motor looks like it was tortured and put away wet! I know I didn't beat it. Not even with a loader. I'm just glad I bought a spare Onan years ago when I first read about the valve seat issue in that motor. If/when this much newer lower hour Onan fails, I may be looking at a different option.

I'm open for suggestions on what to do with this engine guys. Let me have it! Ditch it? Buy a block and transfer? Play with it to get it marginally working? Put a 300 Win Mag through it end to end? 

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sorekiwi

The seat on the rear cylinder doesnt look right, it should look like the front cylinder.

 

A quick and easy check of valve sealing is to poor some solvent into the port and see if it leaks through.

 

Onanparts.com sells oversize seats in +.100 and +.162 sizes.  http://onanparts.com/index.php?main_page=product_info&cPath=1_6&products_id=14

Edited by sorekiwi

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JackC

There are several used blocks for sale on eBay with good seats.  Not sure how much work is involved in transferring the good parts to another block.  Maybe have the block checked first and also have the cylinder walls re-honed?  I did get lucky and bought a used 1989 520H with a good running engine with 650 hours for $450.

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Desko

I dunno it looks fine to me maybe its just the camera making it looks sunk down but ive zoomed in and out and it looks flush with the surface to where it should be... And a block transfer isn't to bad just make sure you have all you gaskets and seals and run taps through all the holes in the cyl block to prevent stripping

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Pickle

It appears the rear cylinder intake valve is open in the picture, giving it an odd appearance. What I quickly notice about this engine is the excessive piston to cylinder clearance.

Edited by Pickle

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WH nut

It appears the rear cylinder intake valve is open in the picture, giving it an odd appearance. What I quickly notice about this engine is the excessive piston to cylinder clearance.

You are right, after zooming in, it does look like the valve is open casting a shadow. The smaller valve seat looks a little questionable also. Can you roll the motor over so both valves are closed and take another picture. Id like another look if you could.

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boovuc

I will do this tonight and take a pic from a better angle using a real digital camera. (TDC on the rear cyl).

Let it be said that it is using a little oil but not enough to cause concern on my part. Doesn't smoke but a very little bit on start up but again, it is another issue along with an oil leak behind the PTO on the crank. (Unless it is an oil pan leak and it's getting whipped up by the air movement).

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boovuc

OK. Here are two new pics of the rear cylinder and that over-sized intake valve and seat at TDC ready to fire. That seat is the same depth as the exhaust valve seat. I did notice that you can spin the valve with a little finger pressure which you should never be able to do at this point in the cycle so it is leaking. I'll see if that can be tightened up when I adjust the valves. Just past TDC the valve is tight against the seat. (Can't spin it). I see what your seeing WH Nut but when I compare it to the front cylinder, they look identical. I had the spare motor apart before installing it on the loader and it looked the same except this valve and seat is a whole lot bigger. The spare on the loader right now tested out at 120 and 125 PSI rear to front. Regardless, I'm going to clean it up, adjust the valves, take apart and wash out the crankcase breather then put it back together using the old gaskets and redo the compression test just to see what I have. I have absolutely nothing to lose and if I can get 90 PSI minimum from both rear and front, it could go back in service temporarily if need be as my spare.

 

 

 

 

 

 

 

 

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WH nut

Ok that looks better. I would say its ok. Spinning the valve while its closed tells me its out of adjustment, which would cause low compression. Get her all cleaned up and adjusted and you should be ok.

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decksetter

I did notice that you can spin the valve with a little finger pressure which you should never be able to do at this point in the cycle so it is leaking.

That's exactly what I've found in 3 different onans. I don't know why, but that rear intake valve works itself too tight to close completely.

You know the valve isn't completely closed when it's supposed to be, so you don't have that pressure down on the valve seat to hold it in place periodically while the engine goes through thermal cycles, to me it's no wonder the seat pops.

Also if that intake valve isn't closing completely you're going to throw some hot exhaust back into the intake. I believe that's why my p220 idled rough but smoothed out immediately when I adjusted the valves. This would also probably cause that dreaded carbon buildup.

What I'd really like to know is why and how the valves tighten themselves and what we can do to prevent it.

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WH nut

That's exactly what I've found in 3 different onans. I don't know why, but that rear intake valve works itself too tight to close completely.

You know the valve isn't completely closed when it's supposed to be, so you don't have that pressure down on the valve seat to hold it in place periodically while the engine goes through thermal cycles, to me it's no wonder the seat pops.

Also if that intake valve isn't closing completely you're going to throw some hot exhaust back into the intake. I believe that's why my p220 idled rough but smoothed out immediately when I adjusted the valves. This would also probably cause that dreaded carbon buildup.

What I'd really like to know is why and how the valves tighten themselves and what we can do to prevent it.

The valve hammer down in to the seat is why they get tight

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