compression issues

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compression issues

Post  jusstsam on November 8th 2013, 3:50 pm

Forgive me in advance for any lack of learning that drives me to these questions. So I have a 460 cobrajet set up. Was burning oil and fouling out a few plugs so I figure I need a valve job. Pull the intake and see it is leaking from bad gasket setting. Gasoline has gotten into the oil and the #4 cylinder head was full of oil gas and water. I was fairly confident I didnt need a valve job anymore and I could get away with cleaning up the intake, valley/pan, change the oil and run it. I swapped out for new lifters and glad I did because they were all frozen but 2. While adjusting lash I compression tested and was fine until I got to 5 and 7.
Cold dry test 155 across but #5 was at 90. #7 over 200. Repeatedly. I am thinking I may have buildup in 7 and the gas in yhe oil may have damaged the rings on #5? Could it be a valve issue as previously thought? Please any advice would be greatly appreciated.

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Re: compression issues

Post  LivermoreDave on November 8th 2013, 4:00 pm

Gasoline in the oil is a killer! If you have lifter issues there may very well be other issues that haven't raised their ugly head yet! May be a good time to check bearings, camshaft lobes and valve guides.

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Re: compression issues

Post  jusstsam on November 8th 2013, 6:09 pm

How does one check valve guides? I havent done this much internal work since high school

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Re: compression issues

Post  kim on November 8th 2013, 6:50 pm

With a guide gauge tool. Various rod diameters as feelers, much like feeler gauges,

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Re: compression issues

Post  jusstsam on November 8th 2013, 9:04 pm

Could a blown head gasket cause the two cylinder s to act this way?

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Re: compression issues

Post  kim on November 9th 2013, 12:13 am

Didn't read initial post, sorry. Rings washed out on 5 cam wiped out on 7. Time for a rebuild.

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Re: compression issues

Post  jusstsam on November 9th 2013, 12:39 am

This is a newish motor. I think its a combo of a bad intake seal, then lifters going out from sitting too long. Then valve guides going out.

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Re: compression issues

Post  bigblok2000ranger on November 9th 2013, 12:58 am

Knock the porcelain out of an old spark plug and tap to 1/4" pipe and add a air compressor fitting and put some air to each offending cylinder with the pushrods removed. If air comes out the intake you have a intake valve bad or out exhaust bad exhaust valve, air comes out of the crankcase=bad ring seal, air out the radiator=blown head gasket or cracked head. Costs about $3 and can give some insight into the issues before you have to tear it down.
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