520 Problems?

[Brad-bunch1978 0]

I got a 520h . Maybe you can help a bit ? I bought mine a couple years back . It had been sitting outside for a while . Didn't run when I bought it so I cleaned the tank an carb . As well as I pulled the flywheel off an cleaned the stater . It runs now but it hits on one cylinder some times . It's real hard to tell unless your mowing or pulling something . Have any clue what may be wrong ?

[Sparky-(Admin) 23,016]

Welcome to Redsquare Brad!

I split your question out of where you originally posted it and made it its own thread. It'll get more attention this way.

Hopefully your Onan twin hasn't lost a valve seat.

Mike............

[varosd 1,185]

pull the valve covers/heads and see if, like Mike stated, the rear seat might have let loose. if OK then adjust valve lash/decarbon the head and bolt back up. how many hrs, what year?

[Brad-bunch1978 0]

It is between 1989 an 1995. How can I tell if it is the valve seats ?

[Brad-bunch1978 0]

A older guy I know who retired from John Deer said something bout valves or valve seats being bad to go bad an was real cheep to fix . He said they put that same motor on John deer lawn tractors as well an that was a normal problem with that motor .

[artfull dodger 440]

when you remove the cylinder heads, you can rotate the engine till each valve is open, then check to see if the seat is loose, usualy will fall right out of the block. or the seat might be loose and holding a valve open, which would cause it to hit on one cylinder. You could also need new plug wires, espicaly after sitting outdoors for awhile. You can get a spark checking tool at Harbor Freight Tools, costs about 10 bucks. Goes inline with the plug wire and lights up when it fires. You can watch to see if the spark is intermitten or not. It fires both plugs at the same time. Is it a P or B series Onan? B series has points. The stator under the flywheel is just for charging the battery and running the Tach. If its a P or Performer series, it has electronic ignition, has a trigger ring under the flywheel to tell it when to spark. I would rule out an ignition issue after it has sat this long. If all the ignition checks out, then move to other tests. Adjusting the valves, done with the engine cold, intake should be .005 feeler gauge and the exhaust is .013. Takes 2 open end wrenches to adjust the lifters. You will need 2 new head gasgets before pulling the heads and decarboning them and the tops of the pistons/valve area. Be carefull where the headgasget seals, you do NOT want to use steel wool ect. A good carb cleaner, parts cleaner tank and a small putty knife is what I use, carefull with the putty knife. A brillo pad swiped from under the kitchen sink when the boss lady isnt looking works well to. I run Seafoam in the fuel to help keep the carbon issue down. The valves and valve seat tends to be more of an issue than carbon, unless its burning oil. Make sure your cooling fins, both on the jugs(front and rear) and around the crankcase are clean and oil free. Fix any oil leaks. Keeping the Onan cool is priority number one! Once you adjust the valves to spec, dont be alarmed to hear them clicking, espicaly when at lower rpm's. Thats normal, not hearing them is a good sign they are out of adjustment. Post up what you find and let me know.

[Brad-bunch1978 0]

I think it a p220

[artfull dodger 440]

P220 is a Performer series 20hp, so electronic ignition. I would check for spark loss, pull the covers over the valves, you may be able to tell if a valve is hung up on a loose seat right then before pulling the heads. Pulling the heads usualy involves taking the engine loose and pivoting it on the mounting plate 90' so you can remove the tin work and remove the heads. A trigger ring giving up or bad spark plug wires would cause it to miss out as well. Does it do it all the time or just when its hot? When its hot points to a ignition coil breaking down. At this point I would remove the hood, makes it so much easier to work on the engine. Remove the drive belt, unbolt the engine, pull heads, inspect, decarbon and go from there. A loose valve seat will need to be fixed by a shop that knows what they are doing. Its not a terminal issue unless its beat the block to death. If you need the retorque pattern and specs, I believe the specs are over in the manual area of RS. Mike

[Shuboxlover 480]

Pull the rear head.....your culprit may rear it's ugly head!!!!!!! If not, you're golden and only out the cost of a head gasket. It is amazing how well the P220G's run on one cylinder!!!!!!!

[546cowboy 302]

The fastest way is to do a compression check on both cylinders. If the rear cylinder is low or has none you have a valve seat loose. Yes it is sometimes amazing how well a P-220 will run on one cylinder.

If one is out it can be replaced by a good machine shop for around $100 to $150 if you pull the motor and head, carb, intake and exhaust manifolds. I have had three of them done that way but one of those needed to have the block welded and then re-cut for a new seat. Obviously not a DIY job unless you are a professional welder and at a higher cost. But considering what another engine would cost you................ :eusa-think:

Edited January 27, 2013 by 546cowboy

[ericj 1,579]

do your self a favor and run a compression check before you go any further !!!!!!if it is between 75 and 125 psi you are with in spec. if at lower end you may want to do a decarbon and valve job but if you go to the manual it will tell you how to check all components of the electrical, coil, ignition trigger, rebuilding carb an a whole lot more. so run the compression check 1st. who knows it could be just bad plugs or wires. i just had to replace the plugs in my 520H that i have my 2 stage blower on. last week i went to start it and it started missing and blowing fire out the exhaust. ran a compression check found 120 psi per cylinder, put new set of plugs in and it even starts better now, blew 2" of snow real good last night and ran like a champ. just my 2 cents worth

eric j

[6wheeler 669]

I'm with ericj on this one. However, on my 91 520 I had the same problem. The culprit was the rear spark plug wire. I robbed the coil wire of the C-165 and finished mowing, I put a new wire on the 520 and it runs just fine now. BTW, since I was in there, I replaced the plugs and both wires. An ounce of prevention and all.

[ericj 1,579]

any updates, how are you making out with your tractor ?

[Brad-bunch1978 0]

Thanks for the heads up on my wheelhouse 520-h . I've just let it sit in the barn an hadn't really even thought bout it in a while. But I gott a email from redsquare. I've talk to other ppl both the those onan engines and not to many people have even heard of them much less know how to work on them. But I have gotten a older friend who retired from John deer an he's told me the valve stems were bad to fall down on the Onan motors as well.