Need help with 520 Onan engine

[byoung36 18]

I picked this up today, I am not familiar with the Onan engines. This tractor had what seem to be a miss. I have been trying to diagnose the issue and have found that the rear cylinder does not have any compression. Is there any know Onan problems that can cause this without it being a broke rod or something of that nature. The tractor only has 598 hours and appears to have been really taken care of. 

 

 

 

 

 

 

Edited July 15, 2019 by byoung36

[WVHillbilly520H 10,376]

5 hours ago, byoung36 said:

I picked this up today, I am not familiar with the Onan engines. This tractor had what seem to be a miss. I have been trying to diagnose the issue and have found that the rear cylinder does not have any compression. Is there any know Onan problems that can cause this without it being a broke rod or something of that nature. The tractor only has 598 hours and appears to have been really taken care of. 

 

 

 

 

 

 

Spinning that big 60" deck it could have popped a valve seat on the rear cylinder but instead of NO compression it would have LOW...preventative maintance like oil changes may not have been preformed like keeping it clean...looks just like a 520 I saw on CL near Roanoke VA for sale last week.

[lynnmor 7,833]

Most likely a valve seat, pull the rear head and then you will know.  To have no compression, the seat would have to be real bad.

[byoung36 18]

1 hour ago, WVHillbilly520H said:

 

 

1 hour ago, WVHillbilly520H said:

Spinning that big 60" deck it could have popped a valve seat on the rear cylinder but instead of NO compression it would have LOW...preventative maintance like oil changes may not have been preformed like keeping it clean...looks just like a 520 I saw on CL near Roanoke VA for sale last week.

 

Thanks, I will look in to it more. This one I bought at an auction Saturday in TN. 

The PO passed away and they sold everything including the house and land. 

 

 

[WVHillbilly520H 10,376]

2 hours ago, byoung36 said:

 

 

Thanks, I will look in to it more. This one I bought at an auction Saturday in TN. 

The PO passed away and they sold everything including the house and land. 

 

 

If you got it cheap enough maybe @richmondred01 has an Onan engine ready to go back in it, there's nothing quite like the sound of one of these 520s up on the governor spinning that deck or a snow thrower JMO... good luck with your new machines.

[1995 520H+96+97 937]

Is the rubber gasket (82-0680) missing around the oil filter?

Drain the engine oil and cut open the oil filter to check for metal.

[oliver2-44 10,738]

I don't have any Onan experience, but I've read on hear many times how important it is to pull the engine to remove and clean under the rear cylinder tins.  Due to it's mounted location the rear cylinder catches a lot of grass, especially from that big deck.  There's threads on hear about cutting louvers or drilling many holes through the belt guard to get air circulation.  Rear cylinder loosing compression seems to come up in many threads.  It seems when the rear cylinder doesn't get ventilation cooling, it overheats and the valve seats come loose.  

Nice looking machine that just needs a good grooming.  Good Luck with it

Edited July 15, 2019 by oliver2-44

[adsm08 3,282]

7 hours ago, oliver2-44 said:

I don't have any Onan experience, but I've read on hear many times how important it is to pull the engine to remove and clean under the rear cylinder tins.  Due to it's mounted location the rear cylinder catches a lot of grass, especially from that big deck.  There's threads on hear about cutting louvers or drilling many holes through the belt guard to get air circulation.  Rear cylinder loosing compression seems to come up in many threads.  It seems when the rear cylinder doesn't get ventilation cooling, it overheats and the valve seats come loose.  

Nice looking machine that just needs a good grooming.  Good Luck with it

 

Since piston rings are sprung metal, and heat is the natural enemy of springs, overheating will destroy a piston ring and cause it to stop sealing as well.