P220G De-Carbon/Valve Adjustment Questions

[1990Onan520-H 15]

Hi guys and gals, I am about to tackle my first de-carbon and valve adjustment on my 88 520-H with 900 some odd hours on it, that is surging/stalling and appears to have in intake leak in the rear cylinder. I have read a bunch, but want to be sure I don't miss anything. I plan to replace the carburetor gasket, intake gaskets, check the manifold to make sure the halves aren't leaking, adjust valves and replace head gaskets as well as exhaust gaskets. I was also debating replacing valve seals and checking for loose valves seats/grinding valves (not lap, grind 😊). My question is how do I do that?

 

I'm assuming replacing the valve seals requires taking the valve spring and keepers off and I don't have those tools for this engine, so I'm open to thoughts/opinions on that if I should bother (engine doesn't smoke while running, just a bit on start up). Second, how do I check the valve seats to know if they are loose and if my valves need to be ground (the engine does make a mild tic and I'm wondering what it may be)? I have 85 PSI cold compression in both cylinders, so I'm sure something is out of wack unless it's just typical wear, but at least they have worn in even. Finally what's the best way to clean the carbon off the piston and head?

 

I'm fairly mechanically inclined and have built a few car engines, just not too familiar with theses Onans. I do have the engine I pulled off my 90 that chucked a rod, that I can practice on if I have to. Also anything I'm missing that I should do/replace while this is apart? It's kind of a pain and I'd like to only do it once.

 

TIA

[oneroadkingman 53]

Most small engines require at least 90 pounds per square inch (PSI) of compression when hot, and 100 PSI when cold. If air is leaking somewhere in the engine, you will notice a drop in compression. Symptoms of a compression problem can include hard starting, erratic idling, loss of power under load and hard starting when the engine is hot. good luck with your project!

[lynnmor 7,823]

You are on the right track.  Start with applying penetrating oil to all fasteners and reapply twice a day till work begins.  The intake and exhaust bolts strip easily, do not force removing or installing, leave the penetrating oil do its job.  You could have the valves and seats ground at an automotive machine shop.  Valve seats can be moved with the valve off of it if they are loose, some automotive machine shops can replace them with oversize seats.  Replace the intake valve seals.  Use Onan intake gaskets, cheap ones can turn to mush.  Check the intake manifold for a cut where a heat shield rubs it.  Leaky valves don't cause smoke, a poor valve seal will cause a bit of smoke at startup.  You can buy or rent valve spring compressors at many auto parts stores, there are many styles and prices, so snap a photo to help you get one that will work.  Some folks can use a pry bar to compress the spring, but I don't recommend that.  Plug oil return holes to keep parts and dirt from falling in.  Use a plastic scraper so that you don't gouge anything while removing carbon, some solvent can help..  A softer wire brush can be used if you are careful.  Glass bead the expensive muffler, weld any cracks and paint with Rustoleum high temperature paint.

[richmondred01 2,307]

It’s not a bad job. I highly recommend you do it sooner then later.

Every wheel horse I ever had with an onan I pulled the engine and decarbed. That way I knew what I was starting with.

After that, every 300 hours I will pull the engine and decarb. I’m down to just one horse with an onan now and it only gets about 25 hours a year so I’m good for another 10 years. 

Like lynnmor said, soak the bolts and take your time removing them because they will strip and the head bolts will break. 

Don't use power/air tools to remove the bolts. 

[1990Onan520-H 15]

What's crazy is the 90 that chucked a rod came right apart. No issues. Figures. I'm sure this 88 will give me issues since it's fairly rusty and spent a lot of time either outside or sitting in dirt. Was also a snow blower machine. So I'll be sure to spray it.

[1990Onan520-H 15]

10 hours ago, lynnmor said:

You are on the right track.  Start with applying penetrating oil to all fasteners and reapply twice a day till work begins.  The intake and exhaust bolts strip easily, do not force removing or installing, leave the penetrating oil do its job.  You could have the valves and seats ground at an automotive machine shop.  Valve seats can be moved with the valve off of it if they are loose, some automotive machine shops can replace them with oversize seats.  Replace the intake valve seals.  Use Onan intake gaskets, cheap ones can turn to mush.  Check the intake manifold for a cut where a heat shield rubs it.  Leaky valves don't cause smoke, a poor valve seal will cause a bit of smoke at startup.  You can buy or rent valve spring compressors at many auto parts stores, there are many styles and prices, so snap a photo to help you get one that will work.  Some folks can use a pry bar to compress the spring, but I don't recommend that.  Plug oil return holes to keep parts and dirt from falling in.  Use a plastic scraper so that you don't gouge anything while removing carbon, some solvent can help..  A softer wire brush can be used if you are careful.  Glass bead the expensive muffler, weld any cracks and paint with Rustoleum high temperature paint.

So a loose seat will literally be able to be moved by hand? Are there any signs its about to happen or anything I can do to prevent it internally while I'm in there?

[1990Onan520-H 15]

10 minutes ago, richmondred01 said:

It’s not a bad job. I highly recommend you do it sooner then later.

Every wheel horse I ever had with an onan I pulled the engine and decarbed. That way I knew what I was starting with.

After that, every 300 hours I will pull the engine and decarb. I’m down to just one horse with an onan now and it only gets about 25 hours a year so I’m good for another 10 years. 

Like lynnmor said, soak the bolts and take your time removing them because they will strip and the head bolts will break. 

Don't use power/air tools to remove the bolts. 

 

Do you find the onans still get carboned up even with modern fuels?

[lynnmor 7,823]

If the seats cannot be moved they should be good to go.  The seats can wear the hole by a large amount.  I find that the carbon buildup is not a great problem, but I run the engines fast for general use, and full throttle for mowing and snowblowing.

[1990Onan520-H 15]

52 minutes ago, lynnmor said:

If the seats cannot be moved they should be good to go.  The seats can wear the hole by a large amount.  I find that the carbon buildup is not a great problem, but I run the engines fast for general use, and full throttle for mowing and snowblowing.

My 90 was fairly clean at 850 hours. Some, but nothing crazy at least from what I saw or what I would consider bad. Again that engine chucked a rod, so I was only taking it apart for fun and to maybe learn a little about how they came apart before ripping into my 88. It didn't look like the piston made contact with the head, so I think it was lack of oil changes that did it in.

 

I'm doing the decarbon on my 88 just to be safe and it also looks like the front head gasket may be leaking a little oil. Heck after 900 hours and 30+ years im sure all gaskets are junk.

Edited August 15, 2019 by 1990Onan520-H

[1990Onan520-H 15]

Does anyone know the part number for the exhaust valve guide seal? I only see an intake seal or am I missing something completely?

 

Planning to buy the below:

 

https://onanparts.com/index.php?main_page=product_info&cPath=1_8&products_id=32&zenid=f26b4942ed2cc39294e29137b040b049

https://onanparts.com/index.php?main_page=product_info&cPath=1_7_19&products_id=18&zenid=f26b4942ed2cc39294e29137b040b049

[richmondred01 2,307]

Onan performer only has the intake seal. 

 

[1990Onan520-H 15]

2 minutes ago, richmondred01 said:

Onan performer only has the intake seal. 

 

Kinda had the feeling someone was going to say that lol. Thanks.

 

Any other seals I should worry about while this is apart?

[richmondred01 2,307]

While the blower housing is off I typically replace the vacuum hose for the fuel pump.

On an engine that I plan on keeping for awhile I pull the flywheel and also replace the ignition module with a new one. 

Thats just me. I tend to sometimes go overboard.

I also have a stash of the modules that I procured years ago. 

Edited August 15, 2019 by richmondred01

[1990Onan520-H 15]

How about carb to intake gasket? I just cleaned the carb up and really only need the carb to intake gasket, not an entire kit, but can't seem to find just that on onan parts. Anyone know if they sell just that one gasket?

 

Nevermind! Found it. I think. I'm assuming this is the right one for the P220G.

 

https://onanparts.com/index.php?main_page=product_info&cPath=1_7_22&products_id=25&zenid=f26b4942ed2cc39294e29137b040b049

Edited August 15, 2019 by 1990Onan520-H

[1990Onan520-H 15]

On 8/15/2019 at 3:16 PM, 1990Onan520-H said:

How about carb to intake gasket? I just cleaned the carb up and really only need the carb to intake gasket, not an entire kit, but can't seem to find just that on onan parts. Anyone know if they sell just that one gasket?

 

Nevermind! Found it. I think. I'm assuming this is the right one for the P220G.

 

https://onanparts.com/index.php?main_page=product_info&cPath=1_7_22&products_id=25&zenid=f26b4942ed2cc39294e29137b040b049

 

I just realized this is only one of two gaskets under the carb. Does anyone know what the two gaskets are for the P220G? I see the carb bolted to some kind of what I guess would be called a flange??? and then the flange bolted to the intake. I want to be sure I replace everything that could be causing air leaks but can't seem to figure out which gasket is which under the carb.

[lynnmor 7,823]

Not sure what you have going on, I do see some engines have two different part numbers for the gasket under the carb.  Can you post a photo?

[1990Onan520-H 15]

6 minutes ago, lynnmor said:

Not sure what you have going on, I do see some engines have two different part numbers for the gasket under the carb.  Can you post a photo?

https://www.ebay.com/c/1428751515

 

Are you able to see that link? If so see how the carb is bolted to a spacer and then spacer is bolted to the intake? There are two gaskets there it looks like.

[lynnmor 7,823]

I see what you mean now.  I have intakes with just the bolts at an angle and no extra "flange", perhaps yours has been replaced with an older model.  Possible gasket numbers are: 154-0733, 145-0438, 146-0499.  You might want to contact Boomer for parts and more information.

[1990Onan520-H 15]

8 minutes ago, lynnmor said:

I see what you mean now.  I have intakes with just the bolts at an angle and no extra "flange", perhaps yours has been replaced with an older model.  Possible gasket numbers are: 154-0733, 145-0438, 146-0499.  You might want to contact Boomer for parts and more information.

Older than 88 you mean? I guess I'll grab the stuff tonight, measure bolt holes and see if I can figure this out. I'll shoot Boomer a message as well. Thanks.