Important info to keep your Onan happy

[artfull dodger 394]

Most guys will discuss the pros and cons of the Onan engines till the cows come home. But to keep these happy, four simple things need to be done.  The most important is making sure your rubber grommet around the oil filter that seals it to the cooling tin work is in place. Most every Onan I see is missing this on garden tractors.  A very simple $6 part, but HIGHLY important to keeping that rear/right side cylinder cool.  Without this little rubber part, you loose 30% of your cooling airflow on that side.  And that figure is from Onan themselves and does not take into account the belt guard helping restrict cooling air flow on our Horses.  Second thing is to do your valve lash adjustment sooner than the book says.  In Onan factory schooling they taught us every 250ish hours vs the 600 in the book was the best practice.  Valves being held slightly open is one the huge reasons we see popped valve seats on engines.  Add in the missing grommet on the oil filter and we have a perfect storm to overheat the engine and superheat the valve seat=popped seat or thrown rods. Three...keep it clean, when you change the oil, clean up your mess, wipe it all down in there, shine a flashlight in there and look, is the seal under the flywheel seeping, or is the jug oil/grime free and dry?  Pulling the left/front side cylinder head tin every spring or every oil change is good practice to check for the same along with rodent nests.  And finally, run them at full throttle at all times after starting and till just prior to shutting them down.  Onans do not tolerate overheating from extended RPM use like a Kohler can.  Just nature of the beast and the design of the engine coming from the generator world.  Once she is running, throttle up before driving off, drop back to 1/2 to 3/4 to engage PTO, then right back to full power 3600rpm + or - 50rpm with the PTO engaged.  Nothing in the small engine world can match the snarl and power of an Onan B or P series, they just take a bit more upkeep to keep them happy.  Mike

[lynnmor 7,303]

Actually every thing that you mention is for any small engine with the exception of keeping a good seal on the oil filter.  Onan generators ran at 1800 RPM all day with a full load, those engines were the same except for the governed speed.  But yes, run them at 3600 RPM when worked hard.  I would add that keeping the fuel system clean so that the fuel air mixture is correct, lean mixtures can overheat valves.  I helped a guy with a Kohler that had burned valves every season because the dealer leaned the carburetor every time he worked on it.

[artfull dodger 394]

yep, and a leaking intake manifold on an Onan will make her lean, hence the surging.  Onans are just much less tolerant of anything out of spec than a Kohler.  The valve adjustments are unique to the Onans in the older flat twins.  Kohlers you set via grinding when the engine is built or rebuilt with new valves.   Wasnt till the OHV engines of modern day did we see adjustable valve lash return and become a service item that needs checked.   

[bds1984 1,429]

On 9/4/2024 at 12:48 PM, artfull dodger said:

yep, and a leaking intake manifold on an Onan will make her lean, hence the surging.  Onans are just much less tolerant of anything out of spec than a Kohler.  The valve adjustments are unique to the Onans in the older flat twins.  Kohlers you set via grinding when the engine is built or rebuilt with new valves.   Wasnt till the OHV engines of modern day did we see adjustable valve lash return and become a service item that needs checked.   

 

I've done the grinding trick on many engines, but my K341 has adjustable lifters on it.  Get them out of adjustment too much and you'll hear it.  

[artfull dodger 394]

They will tap if they are out of adjustment to the wider side.  Sadly Onans close up usually, not the other way around.  You want to hear an Onan being a bit tappy when its idling.  New OHV engines are also adjustable.  Briggs are bad for going wide and loosing the compression release making the starter unable to roll them over.  I remember Kohler had a Command series that had hyd lifters that were self adjusting.  Was one of the noisiest clacking set ups they ever came up with.  Had plenty of customers complaining about the racket they would make and swearing they needed to be adjusted.  Not one of Kohlers better ideas.