Compression problem

[twodogs 55]

My 520h has been running a bit rough the past couple of weeks, so I decided to check the compression on both cylinders.  The front is showing about 85 psi, whereas the rear has a paltry 30 psi.  Of course, I'm assuming it's blown valve seat, but my fingers are crossed that it's something else.  Just 60 hours ago (per the tractor's hour meter), I performed the following maintenance (and it was running like a top after it):

 

- new intake and exhaust manifold gaskets

- new head gaskets

- new valve box gaskets

- cleaned the crankcase breather

- adjusted intake and exhaust valves/tappets

- decarboned the heads

- new plugs

 

Aside from tearing the engine apart to determine the problem, does anything come to mind that I should look for as I head down this rabbit hole?  (I'm a novice at this stuff but have been pretty successful so far on the DIY aspect).  Thanks for any insight (and condolences?) you can provide!

 

[953 nut 55,223]

After the service that you performed on the engine did you bring it up to operating temperature, let it cool off and then retorque the head bolts? That is an old school method that seems to not be mentioned in service manuals but is important.

       I just looked at the Onan Service Manual and they call for a retorque after 50 hours. I do it after a five minute warm up.

Edited July 16, 2022 by 953 nut

[lynnmor 7,305]

3 hours ago, 953 nut said:

After the service that you performed on the engine did you bring it up to operating temperature, let it cool off and then retorque the head bolts? That is an old school method that seems to not be mentioned in service manuals but is important.

     I just looked at the Onan Service Manual and they call for a retorque after 50 hours. I do it after a five minute warm up.

 

While it may be a good idea to re-torque the head bolts after the engine has been run, the manual does not specify that for the P220 engines.  The P224 engine had studs, nuts and compression washers and they are to be re-torqued after 50 hours.

 

I wonder if @twodogs checked for loose valve seats and burned valves while the heads were off.  Running an engine with a fouled carburetor causes a lean mixture and it can lead to burned valves or worse.

[rmaynard 15,458]

Also, after 60 hours of running (cutting grass especially) the air flow around the rear cylinder (which is poor to begin with) can be compromised by the accumulation of grass clippings. This is a common problem. This potential overheat condition will burn a valve seat that was otherwise okay upon last inspection. Hope that's not the case, but it's the first thing that comes to mind for me.

[twodogs 55]

As you likely surmised, it was the intake valve seat.  I decided to just remove the engine completely so I could work on it at bench level (and in the event it would have to go to a machine shop).  I was also pretty certain it was the valve seat, so I figured I would save some labor costs by doing that work myself.  Now that I know at least part of the problem, I'm hoping you all could give me a little guidance on my options.  Do folks typically take the engines to a machine shop to see about getting it rebuilt?  I'd like to keep it going if at all possible.  I really rely on it with the 60" mower, and the plow to clear snow in the winter.  I'd love to hear what you might consider for a rebuild/repair punch list.  Thanks folks!

 

[lynnmor 7,305]

Unless you have a machine shop the valve seat portion of the repair will need to be farmed out.  It would be best if you have the shop do a complete valve job on all four valves and have new seals put on the intake valves.

 

You may not need a complete overhaul which would be costly, you and the shop can take the best guess as to what is needed.

[twodogs 55]

1 hour ago, lynnmor said:

Unless you have a machine shop the valve seat portion of the repair will need to be farmed out.  It would be best if you have the shop do a complete valve job on all four valves and have new seals put on the intake valves.

 

You may not need a complete overhaul which would be costly, you and the shop can take the best guess as to what is needed.

I don't have a machine shop (nor that kind of expertise) so I'll definitely need to have someone else do the work.  I figured a complete valve job would be called for, and I'm just hoping that's all they find.  This engine has never burned oil, and I always check it before startup.  I guess I'll be calling around on Monday to see who can do the work.  On that note, how critical is it to find a shop that works on Onan engines?  I don't have a lot of options out here, so I'm hoping a reputable shop is all I'll need to find.  I appreciate your input.

Edited July 17, 2022 by twodogs

[Pullstart 62,852]

We have a local NAPA distribution hub that has a machine shop for reasonable rates.  Maybe check with some auto parts suppliers for suggestions on local shops that could help?

[lynnmor 7,305]

A valve is a valve, so any shop that can accommodate the size, valve stem angle and shape of the engine will work.  Many shops no longer want to be bothered with small engine work, so you might have problems finding one.  We have a NAPA in York, PA that did these engines, I don't know what the current status is.  As you call around, ask how much of a press fit they will use to hold the seat in place, it should be about .0055", their answer will give you an idea if they are qualified.

[twodogs 55]

6 hours ago, lynnmor said:

A valve is a valve, so any shop that can accommodate the size, valve stem angle and shape of the engine will work.  Many shops no longer want to be bothered with small engine work, so you might have problems finding one.  We have a NAPA in York, PA that did these engines, I don't know what the current status is.  As you call around, ask how much of a press fit they will use to hold the seat in place, it should be about .0055", their answer will give you an idea if they are qualified.

Thanks for that info; that’s helpful. One more question. Will I need to pull everything off the engine in order for a shop to do the work? I’m hoping it can be done with the flywheel left on, but with the way this thing has been going, I’m preparing for additional work on my end. 
 

Best birthday, ever!

[lynnmor 7,305]

27 minutes ago, twodogs said:

Thanks for that info; that’s helpful. One more question. Will I need to pull everything off the engine in order for a shop to do the work? I’m hoping it can be done with the flywheel left on, but with the way this thing has been going, I’m preparing for additional work on my end. 
 

Best birthday, ever!

 

You should talk to the proposed shop regarding more disassembly.  I had the heads off and clamped the engine to the bed on a Bridgeport milling machine, it took every inch of height. 

 

The flywheel is very heavy but it can be pulled off easily with a homemade puller, mine is just a steel plate with 4 holes to match the flywheel and a tapped hole in the middle.

 

 

[twodogs 55]

I went to four shops yesterday with the engine, and that was an exercise in futility.  Three of them either had no interest in doing the work, or stated that the engine wouldn't fit in their machine.  And the remaining shop said that I should strip it down to a bare block and they would think about it (again, about zero enthusiasm).  I looked at the block again where the valve seat resides, and it's clear that the block was hammered pretty hard by that loose seat.  I think that portion of the block would have to be built back up and then routed out for a new seat.  If I had to guess, that seat hammered it 40 thousandths deeper into the block.  Still can't believe I did it!  

 

I just made phone inquiries with shops in other towns that are hours away from me, and every one of them declined the work even before seeing it.  At this juncture, I'm wrestling with re-powering with a different engine, or just selling the 60" mower deck, snow plow, and tiller.  I know I have a rear axle issue that I'll need to address if I keep it.  When I have the rear end lifted up, I can move the wheel back and forth, as if there are wheel bearings going out.  Decisions, decisions...

[lynnmor 7,305]

Here is an oversize seat, It will require a knowledgeable machine shop to install.  As you found out, shops that can actually do things besides sell parts are getting scarce.

 

You could try placing an ad in the wanted section to see if you can find a block or anything up to a complete engine.