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Onan CCK Low Compression

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Old 09-13-2017, 04:14:37 PM
cocoabill cocoabill is offline
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Default Onan CCK Low Compression

I have an old Onan CCK which has run great for the 15 years I have had it. After last years hurricane season I decided to put new brushes in and change the old sludge oil that had been in there forever. Well this hurricane hits and it wont start.

Looked at everything, it has a decent spark no fuel sediments, it would not really pop but some black smoke would slowly emerge from the exhaust. After thinking of everything it finally hit me that I took out the old oil and put some wonderful Mobile 1 5-30 synthetic oil in and maybe I lost compression due to this. I checked the compression today and its 50 and 55, so I think I was right, it probably wasn't good before but I think I am lower now.

My question is am I too late to save this or should I get some 50 weight oil and see if I can get compression back? Anybody have any other tricks?

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Old 09-13-2017, 04:48:15 PM
Kevin K Kevin K is offline
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Default Re: Onan CCK Low Compression

I hope you cleaned any sludge out of the oil pan prior to using the nice new synthetic oil.

Your low compression is probably caused by carbon sticking under the valves. A leakdown test would point you in the right direction. I would do this before pulling the heads to remove the carbon as it would tell you if the problem is rings or valves. If you have the older metal clad asbestos head gaskets, you can reuse them.
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Old 09-13-2017, 08:13:22 PM
len k len k is offline
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Default Re: Onan CCK Low Compression

On old cars we'ld add a can of STP (oil treatment), but 50 psi is right on boarder of not starting.

On other small engines I've reused even graphite head gaskets. Don't take them apart hot or they supposedly like to peel.
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