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Discussion Starter #1
Pulled the plugs to check for spark and discovered no compression in one cylinder. No knocking or banging when turning the engine over. Does have spark and finally started but is only running on one cylinder I am sure. Why no noise? I have had other engines break a rod but always hammered. Would a valve cause complete loss of compression? Do these Onans have compression releases? The bad cylinder is closest to the steering wheel and I assume this means I will have to pull the engine to even get the head off to start looking. Anyone have this happen to them or any suggestions on what to do or look for next. Winter is fast coming and this was to be my plow tractor. :crybaby: Any help is really appreciated.
 

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I don't know much about the onan engines but I think that it is common for them to drop a valve seat. This would cause the valve to stay open which would result in no compression.
 

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Yes, if valve is stuck open or seat came out, you will have no compression. You may still feel/hear air moving, but it will not hold any psi. I know nothing about the Onans, but the Tecumseh OH160, maybe the 18's too are notorious for dropping valve seats, repairing one now myself.
Can you get the valve cover off to see if valves are moving? Can you see inside spark plug hole to see if piston is moving?
 

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pappy
NO comp release on onan.
the cylinder, closest to you is #2.
HE is the LAST guy on the food chain.
if you only have 3 to 4 oz. of oil in the pan,
he gets squat.
this is the cylinder that breaks the rod 99.999 percent of the time.
thank you. boomer. (the used onan engine parts guy)
 

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My vote is with Boomer on this one, with Onan engines that have no compression on #2 it is usually a rod. The good news is they typically break down low towards the crank and do not hurt the block. There is a good chance your crank will be badly damaged though. If this is the case, I would look at converting the engine to a 48 ci unit, (B48 or P218/220 crank, straight beam no window rods, bore out to "standard" Onan bore for Standard pistons, T260G cam, B48G heads, larger crank vent, B48G intake, big throat Nikki carb, big air cleaner, oil filter) 20+ HP!

Joe
 

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you can try dripping some Marvel Mystery down in the intake and exhaust ports and let it sit for a few days, turn it over, put more oil in, etc I had one motor with a collapsed lifter, had been sitting outside for 5 years or so, kept dumping Marvel Mystery Oil in the intake and exhaust and finally it let go, and ran great. I sold it in an 18/6 I restored. Still running as far as I know
 

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Discussion Starter #7
Sounds like I may as well pull the motor. Really can't tell much while it is in the tractor. Not enough room to get the head off number 2. Wish I wouldn't have sold my blade for my Allis CA, then I would have all winter to fix the Sears, now I need to get something fixed for plowing snow. Thanks for the help guys.
 

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onan 18 (joe)
the wide rod, with no window,
still breaks, when not lubed.
the BAD thing, it breaks near the wrist pin.
then the rod tears up everything inside the block.
thank you. boomer (the used onan engine parts guy)
 

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Boomer,

I am aware that the wide rods with no window will break with no lube, anything will, I just figured that if you had to buy new rods anyways, why not get the best ones? Also, every blown engine I have ever tore down the rod broke near the crank where the window is. Do you mean on the later rods they blow up at the wrist pin, or in general? Also, you beat me to the punch on the collapsed lifter thing, maybe James had a stuck valve?

Joe
 

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joe and club
yes the solid rods tend to break near the wrist pin.
NOW, that is the weakest point.
SO, now you have the rest of the rod flying around in the crankcase.
THAT does LOTS of damage.
joe poss stuck valve yes.
also your parts are ready to ship.
thank you. boomer (the used onan engine parts guy)
 

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Discussion Starter #12
Sure hope I am not getting in over my head on this. I was a small engine mechanic 40 years ago and have even done a few B/S since then but not for awhile. Not sure I have all the tools I will need but have a few friends to borrow from. I am counting on all you experts on here to get me through. I am not looking to do a complete rebuild but rather get it running. Money is a problem so I will be in the market for used parts. Thanks for any help I get.
 
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