20hp Onan Engine

[wheelhorsejoe 4]

Onan's engine are new to me,I was wondering if the engine[1992/20hp] has a bad valve seat can it be replaced, can I replace it myself or does it need to go a machine shop, is it worth the cost. The problem with the engine is that it starts to smoke after running for about an hour. Any input is appreciated.

Thanks Joe

[sorekiwi 761]

If that is whats wrong with your motor, the block will need to be machined for an oversize seat. Onan makes an oversize seat, and Onanparts.com has a bigger oversize seat than the factory one.

Check out http://www.onanparts.com/, there is an article there about the seats.

A machineshop will have to do the machining, but the whole engine doesnt have to come apart, just the heads, shrouds and ancillaries.

"Wheelbearing" had a thread recently about reabuilding an Onan, with lots of pictures.

[wheelhorsejoe 4]

Thanks, you said if that the problem, what else can cause this,broken ring?

Thanks Joe

[woodchuckfarmer 333]

I had both intake seats done on my 520 last summer . The $130 was well worth it. It runs like a top now

[sorekiwi 761]

Thanks, you said if that the problem, what else can cause this,broken ring?

Chances are that your problem is not the valve seat coming loose.

A loose valve seat causes a loss of compression on that cylinder. I think thr problem would be there even when the motor is cold.

Because Onan engines have had problems with valve seats coming loose, everyone tends to assume that any problem that raises its head is valve seat related. (Myself included, I bought my 520 running on one cylinder. I assumed it was a valve seat, but came to find out it had a broken rod. The valve seat was fine).

I would do the normal diagnostic routine - compression test, clean and clear breather etc. If that doesnt reveal anything maybe its time to pull the heads and decarbonise. Have a look at the bores while the heads are off.

How many hours are on this motor?