Onan compression problems

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  #1  
Old 02-05-14, 09:45 AM
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Onan compression problems

Hi, I've got a Steiner 420 tractor with an Onan P220 engine. It's extremely hard to start when cold. Once it's running and warmed up it runs fine. I checked all the usual suspects, good spark, choke works fine. I checked the compression, and found that when cold one cylinder is very low (25 psi) while the other is over 100. Once warmed up, both cyls are over 100 psi. Now the really strange thing is that I've checked the cold compression several times, and sometimes the low cylinder is on one side, and sometimes it's on the other side! I checked the valve clearance adjustments and found one intake valve with very little clearance. I adjusted it to spec. and continue to have the same problem. I've read about valve seats coming loose on early 90's Onans (what I have). Could this be the problem? This engine was supposedly rebuilt several hundred hours ago- has about 1400 hrs now. Any Onan experts out there? Thanks, Steve
 
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Old 02-05-14, 04:50 PM
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The only thing I can think of that would cause intermittent low compression is loose valve seats. Doesn't seem likely, but it's possible. Usually loose valve seats make themselves known by intermittent popping through the exhaust or carburetor. I have to wonder if the compression gauge is accurate.
 
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Old 02-06-14, 09:39 AM
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compression

Thanks for the reply. My compression gauge is a good quality one that screws into the spark plug opening. I use it fairly regularly and it's always been OK. I'm afraid the loose valve seat diagnosis is the answer. Steve
 
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