520H - Onan Smokes on Startup

I've been working on my basket case 1993 520H. This is a high-hour, beat up tractor. Everything is loose. I'm trying to make up my mind on what to do with it.

Here is my next problem. The Onan P220G engine runs like a top, but it smokes (blue-oil) like a crazy when I start it up. Once it warms up - no smoke. Then it purrs like a kitten. If it were an over head valve engine, I would say valve seals and guides. But this is a flat head. I will be adjusting the valves, de-carbonizing the heads and other maintenance items. But what could be the cause of oil burn on start-up. The only thing that makes sense to me is rings. The PO says the engine was rebuilt not too long ago, but who knows.

I have to make a decision, do I tear into this or just run it until dies. ??

I doubt it will be rings. When rings are bad it smokes white all the time or when under load. Its probably just one cylinder. I'd check the head gaskets, valve steams and valve guides. My guess its valve guides. Most people, when rebuilding engines, either forget or don't know about them. There more than likely to be bad. Maybe even the valve stems. The reason why I think this is when you start it, it smokes until its HOT. When its hot valve guides will expand, making it so it won't smoke when hot.

I'd try to fix it, if it runs good, which is rare with onans (non adjustable carbs) then I'd do it.

But take the heads off (if you do you will need head gaskets) if there is carbon by the valves, or on the head, its probably guides. If there is a ring on the face of the piston where there is no carbon, but the rest is carboned its rings. But I don't think it is. (if rings are good then the piston should have a thin layer of carbon covering all of the piston, thats normal, its just from the gas exploding)
Hope this helps.

I agree with you Justin. It sounds like valve guides and seals to me. Its been too cold to work on the engine, but that will be changing. My first step will be a compression check, If that is OK, I'll pull the heads and see what I have. With the amount of smoke this thing makes on start up, the cause should be obvious.

I'm with you on non-adjustable carbs. They may work on new engines, but on old tired ones you have no chance. When I bought this tractor I knew it had problems, so did the seller. So I got it for the right price. Even if I re-power it, I should have a decent tractor at an OK price. I bought for the gear reduction steering, I was going to transplant it to my 88 520H and use the rest for parts. But this engine runs so good I'll attempt to save it.

Thanks for the help!

It finally warmed up a bit, so I did a compression check. Warm engine, WOT, both cylinders 130 psi. So no ring or valve problems here. I think I'm going to try valve seal and guides and see what happens. I might be a while until I get to it. I'm working on tightening up the steering and drive linkage and repairing a badly worn belt tensioner shaft - the shaft has worn into the tunnel about 3/4" I've got used tunnel to replace it. Getting down to the tunnel is no easy task.

Thanks for the help!

Ok sound good Kurt. Glad I could help!