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Onan 16hp Twin Question

K

Kerwin

Member
I have a Sears lawn tractor with a 16hp Onan twin that I bought this summer to blow snow.
I've used it several times and have had the problem that after it runs for a while the engine runs rough at high speed/under load.

I've had the air cleaner off during one of these spells, and it appears that it's dumping in too much gas and actually flooding it out. Run, dump gas, flood out/lose power, run, dump gas, etc. It will usually correct itself if given time or shut off and started another day. If I lower engine speed during a flooding spell, it seems to smooth out and run fine.

Any thoughts from you Onan experts? When it runs it is a great tractor and really blows the snow.
 
Maybe the carburetor float level is set too high or the float valve is hanging open or there's too much fuel pressure.
 
Your carburetor float may have a pin hole allowing gas to enter the float. If it is your float won't shutoff the gas that's flooding the engine. Could be dirt under the needle valve that's attached to your float. Hal
 
Hi Kerwin it is could be the fuel pump is over powering the needle seat at hi R's causing the flooding you could also try leaning the main jet just a bit to see if that helps. You might ask your question on My Tractor forum also as there aqre a lot of guys there that have an know the Sears / Onan tractors
GB in MN
 
If what you are seeing is gas spitting back out of the carb, it may not be flooding. Instead a valve may be sticking or set too close allowing combustion/compression pressure to leak back into the intake and out the carb.

If you don't know when it was last done, I recommend starting with a valve adjustment and decarboning. Pull the heads, intake and valve covers. Clean everything well, adjust the valves and put it back together.

Before starting that though, check the intake manifold for leaks. They sometimes leak air at the intake port or between the manifold halves. If so, checking before removal will allow you to fix that while it's apart.

Good luck, Onans are great engines!
 
I would also check intake valve clearance but first I would add two cycle oil 50/01 to gas this will fix valve sticking in guide
traper
 
kerwin.
i think ken is very close here,
but i will add you may have the original
DREADED plastic/foam float.
it does not like our modern fuel. ( ethanol )
it swells up and sticks.
also i would do a compression and leak down test.
thank you. boomer ( the used onan engine parts guy )
 
Start simple... Replace the plugs if you haven't already.

Had a mower doing exactly what you're describing. New coil, ignitor, pulled heads, clean carb, all day and $100 later still dieing out, blowing back through the carb. New plugs and all was well.

Lesson hard learned.
 
It seems like you are describing a lean-run condition. where the engine runs in surges. You probably need to thoroughly clean the carburetor.

Like someone else suggested, if the valves have never been adjusted, you need to do a valve adjustment too. If those valves are allowed to get too much clearance they hammer the valve seats loose.
 
If you have the down draft carb. Remove the top and see if it is flat and level. I have seen them warp. Which will cause leaks and effect the float.





Onan Tech since 1980
 
I had two Fords and they both always started easily and ran good except if you stopped for coffee or go to the bank. They were hard to start and I was told they had composite floats and would get gasoline soaked causing the flooding.
New floats fixed the problem. Hal
 
My brother just went through that with his 318 JD, of all things he put in a new set of points. He says it runs better that it has since he bought it.
 
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