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Inconsistent compression on 1 cylinder #3096660
11/23/22 10:14 PM
11/23/22 10:14 PM
Joined: Aug 2006
Posts: 1,349
warfordsburg, PA
dirt Offline OP
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dirt  Offline OP
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warfordsburg, PA
My stock 1971 360 recently developed a bad miss. I did a compression test and all cylinders are 155 psi except #7 first it was 145 then 100. It is different each time I test it while the others stay the same. I am guessing it is a valve issue. How can I find out though?

Re: Inconsistent compression on 1 cylinder [Re: dirt] #3096667
11/23/22 10:26 PM
11/23/22 10:26 PM
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Bend,OR USA
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Cab_Burge Offline
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Squirt a little oil into the cylinder through the spark plug hole and spin it over before taking the test again, do the test several times without removing the gauge if it is the screw in type twocents scope
Please let us know your results wrench


Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
Re: Inconsistent compression on 1 cylinder [Re: Cab_Burge] #3096673
11/23/22 10:45 PM
11/23/22 10:45 PM
Joined: Aug 2006
Posts: 1,349
warfordsburg, PA
dirt Offline OP
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I put a little oil in and I keep getting 120-125 on that cylinder now.

Re: Inconsistent compression on 1 cylinder [Re: dirt] #3096677
11/23/22 10:57 PM
11/23/22 10:57 PM
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Kirkland, Washington
Pacnorthcuda Offline
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Kirkland, Washington
Cracked ring?

Re: Inconsistent compression on 1 cylinder [Re: dirt] #3096690
11/24/22 12:08 AM
11/24/22 12:08 AM
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Ambridge, Pa.
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rickraw Offline
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Ambridge, Pa.
Do a leak down test. Your problem will show.

Re: Inconsistent compression on 1 cylinder [Re: Pacnorthcuda] #3096691
11/24/22 12:10 AM
11/24/22 12:10 AM
Joined: Dec 2005
Posts: 11,408
Fulton County, PA
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CMcAllister Offline
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Fulton County, PA
Not sure being down 25 or 30 PSI would cause a "bad" miss. I would leak it. Maybe take a look under the valve cover.

A head gasket failure between 5 and 7 will cause strange things to happen.


If the results don't match the theory, change the theory.
Re: Inconsistent compression on 1 cylinder [Re: dirt] #3096716
11/24/22 01:35 AM
11/24/22 01:35 AM
Joined: May 2019
Posts: 702
Lake Villa Il
INTMD8 Offline
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- If it was mine I would just take it apart.

If you want more info before doing so, get it running again, confirm 7 misfire by pulling that plug wire while running to confirm there is no change.

Check intake vacuum and likely observe rapidly bouncing needle due to valve sealing issue. If no broken valve springs, pull the heads. JMO


69 Charger. 438ci Gen2 hemi. Flex fuel. Holley HP efi. 650rwhp @7250 510rwtq @5700
Re: Inconsistent compression on 1 cylinder [Re: INTMD8] #3096726
11/24/22 02:53 AM
11/24/22 02:53 AM
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Fulton County, PA
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CMcAllister Offline
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Originally Posted by INTMD8
- If it was mine I would just take it apart.




That would be the next step.

But, I would still leak it and try to determine where it's going so I have an idea of what to expect to find. Even if you just use an adapter and pump some air into it.

Last edited by CMcAllister; 11/24/22 02:54 AM.

If the results don't match the theory, change the theory.
Re: Inconsistent compression on 1 cylinder [Re: dirt] #3096758
11/24/22 10:25 AM
11/24/22 10:25 AM
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Missouri
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jwb123 Offline
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Missouri
I agree with a leak down test. a loose valve guide will act like you are describing. As you crank the engine for the compression test the valve is jumping around on the seat. The other issue might be since it is a 1971 vintage engine is valve seat recession from not having leaded gasoline. Not sure if you use an additive or not.

Re: Inconsistent compression on 1 cylinder [Re: jwb123] #3096763
11/24/22 10:55 AM
11/24/22 10:55 AM
Joined: May 2005
Posts: 1,944
Apollo, PA.
B1MAXX Offline
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Apollo, PA.
First thing I thought of was a bad lifter.I'm not saying that all the other suggestions.are not good, a leak would be good/easy. After that I would run it with the valve cover off and observe those valves carefully looking for something different about those ones, then take my thumb and push back/down on the rockers at the pushrod seat. Feel a couple good ones then the ones in suspicion. When things are right it will feel smooth a bad one will be harsh or feel like a smack or shock.

Re: Inconsistent compression on 1 cylinder [Re: B1MAXX] #3096878
11/24/22 05:12 PM
11/24/22 05:12 PM
Joined: Aug 2006
Posts: 1,349
warfordsburg, PA
dirt Offline OP
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After I put a little oil in the cylinder yesterday it held at 145 psi every time. Same today. I cleaned the carbon build up off the sparkplug and fired it up. It runs good again. Maybe some carbon wedged a valve open or something.

Re: Inconsistent compression on 1 cylinder [Re: dirt] #3096941
11/25/22 12:55 AM
11/25/22 12:55 AM
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Bend,OR USA
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Cab_Burge Offline
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Bend,OR USA
Thanks for updating us, I hope your problem was as you suggested luck up


Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)






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