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COMPRESSION PROBLEM?  Rate Topic 
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 Posted: 06-30-2005 11:26 am
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BILLBEN
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Joined: 06-28-2005
Location: WINDSOR, Connecticut USA
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I have been unable to get adequate compression on 2 of the 4 cylinders on my 1974 J.H.

This happened after I attempted to change the timing belt, which I have done successfully on 2 previous occasions but this time ended up, accidentally, moving the crank and both cams as the belt was coming off. As a 1974, the book calls for 110 degress on intake and exhaust. But no matter what combinations I try, regarding the number of degress before or after TDC, or 110 degrees vs. 115 degrees intake and exhaust on the cams in any combination, compression on cylinders 1 and 4 consistently read between 110 and 130 lbs. and on numbers 2 and 3 from 60 to 75 lbs.

Knowing it is an interference engine, I have always turned the crank by hand before using the starter, yet it seems I must admit that there are damaged valves on cylinders 2 and 3. Does anyone have any other thoughts or suggestions?!? After many, many hours, I am now at a total loss.

Thanks, Bill

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 Posted: 06-30-2005 05:21 pm
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Mark Rosenbaum
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Joined: 03-12-2005
Location: Kingman, Arizona USA
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If you had good compression on all four cylinders before the belt change, then yes, the valves most likely are bent.  This can be proven or disproven if you set a suspect cylinder to the top of its compression stroke, fill it with compressed air, and listen for any faint hints of air leaking out the intake or exhaust.  If you have a leak at either, or both, then some or all of the valves in that cylinder are not sealing.  The only cure is to remove the head and replace whatever has gone bad.

OTOH, if you hear air coming only from the crank case, then the rings are leaking.  Here, you may have worn liners and/or worn piston rings, or perhaps the rings just stopped sealing due to irregularities in the bore.  The former requires pulling the engine and performing a partial overhaul, while the latter generally requires about 200 miles of driving to re-seat the rings.

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 Posted: 07-01-2005 10:29 am
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Ron Earp
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Location: Cary, North Carolina USA
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Bill,

If you had compression before you did some fiddling, and you've got less now, then it looks like you have a bent valve(s).  The rings and liners didn't get worn or unseated with just your small amount of work and turning the engine over while changing the belt. 

I suppose I'd very carefully go over my cam timing again, squirt some oil in #2 and #3, and then check compression again.  If it comes up low again then it looks like you'll be in for some engine R&R.

Ron

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