Old school Onan

[blurv500]

The equipment in question is a 25 hp Onan NHC from around 1979 that lives in the back of a skid-steer.

One blistering hot summer afternoon I was doing some hard digging- oh baby! that engine was hotter than a two dollar pistol on the 4th of july, and for reasons unknown, the oil seperator part of the crankcase breather apparently filled up with oil, which got sucked into the carb as a slug,and I think momentarily hydrolocked the motor. You should have seen the smoke!

The engine still runs- I use it every day, but it clanks terribly when cold, uses lots of oil and, with only 40-50 psi compression, is noticeably down on power. I believe the rings are broken, I guess could even be a rod bent a little. I'd like to find out, but I'm afraid that if I tear it down, I'll not find enough parts available to put it back togther again. Downtime is an issue.

The questions:
1- Do you agree with my assumptions/diagnosis in the first place?
2- What would cause the oil seperator to fill with oil like that (The ccase was not overfull)
3- Considering the parts availability situation, would you try to fix this or repower with something else. (like a used Wisconsin v4- parts for them seem dirt cheap and easily available.)
4- What is a good (affordable) parts source for these old Onan engines. There's not much for them even on ebay.

Thanks for any information.

[motorhead64]

You will need to do some diagnostic testing to determine your engine's problems. You should have 100 psi compression...you are well below that. Are both cylinders reading low? Your overheating may have caused you to blow a head gasket...or, your valve clearances may be way off. I would check them first. If you ran hot for a long time and it was smoking badly, you may have burned bearings and your rings may have scored your cylinders. MH

[KE4AVB]

Considering parts availability and costs you might be better off re-powering it. The engine is over 30 yrs and sounds like has given you many good years of service but it is now probably time to retire it.

[RoyM]

X2 Onan parts are very expensive, probably more than a replacement Briggs or Kohler engine, then there is the cost of machining. You will need to do some careful measuring, an OHV twin may not fit.

[blurv500]

Thanks for those inputs... when the pros don't know where to get parts, i guess it's time to let her go. The carb is shot (worn throttle shaft) so it doesn't idle down right, and the alternator hasn't charged in years (charge battery after using)- so come to think about it, it'll be nice to kill all those birds with one stone.

[Carlw]

Thanks for those inputs... when the pros don't know where to get parts, i guess it's time to let her go. The carb is shot (worn throttle shaft) so it doesn't idle down right, and the alternator hasn't charged in years (charge battery after using)- so come to think about it, it'll be nice to kill all those birds with one stone.

You can contact your Cummings dealer in your town and see about anything you want for ONAN. And yes, they are expensive. As already noted might be cheaper to change to another brand of engine like Kohler or KAW. Carl