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Hot Rods Compression on one cylinder is different every time

Discussion in 'The Hokey Ass Message Board' started by praisethelowered, Jul 15, 2018.

  1. praisethelowered
    Joined: Aug 14, 2003
    Posts: 1,103


    SBC in my 63 chevy - I get consistent readings on other 7 cylinders but number 5 is all over the place. I have tested it wet, dry, etc. its always low but I expected a consistent low number, then a higher number with oil in the cylinder, so I could know if it is valves or rings.

    Anybody experienced this?
    chryslerfan55 likes this.
  2. doyoulikesleds
    Joined: Jul 12, 2014
    Posts: 295


    do a leak down test sounds like a valve guide/seat problem some times it hits better than others giving you odd readings or it could be a cam lobe
  3. sevenhills1952
    Joined: Mar 14, 2018
    Posts: 956


    A leak down tester would show what's going on. With #5 in valves shut position you'll know whether it's intake, exhaust or past rings.
    Less expensive is the spark plug hole to shop air adapter hose, just thread it in, valves shut you can tell what's going on.

    Sent from my SM-S320VL using Tapatalk
  4. praisethelowered
    Joined: Aug 14, 2003
    Posts: 1,103


    I just tried that and it wont hold air but I can't tell if it is going past the valves or the piston- how would you know this?

  5. razoo lew
    Joined: Apr 11, 2017
    Posts: 494

    razoo lew
    from Calgary

    Gotta be going somewhere:
    - past the rings should show up at the oil fill cap or crankcase vent.
    - past the intake valve should show up at the carburetor.
    - past the exhaust valve should show up at the exhaust.
    clunker likes this.
  6. praisethelowered
    Joined: Aug 14, 2003
    Posts: 1,103


    Yeah, I thought there might be a trick I am not thinking of - I am listening, covering, etc but can't tell where the air is going
  7. Pull the header back. Put some masking tape over the port. Take the carb off. Do the same thing
    19Eddy30 likes this.
  8. 19Eddy30
    Joined: Mar 27, 2011
    Posts: 1,354

    from VA

    Most likely
    One of the 3
    Intake side
    Exaust side
    Crank case
    Witch all requires pulling of the head to fix .
    By chance it could be a lifter on that cylinder is why compression different each time
    Last edited: Jul 15, 2018
  9. henry29
    Joined: Sep 5, 2007
    Posts: 2,837


    Whats the lowest and the highest # on 5?, do you have the valve cover off and watching the valves?
  10. praisethelowered
    Joined: Aug 14, 2003
    Posts: 1,103


    Embarrassing that I needed such common sense info as "tape up the holes" but I did. That worked and told me I have a bad exhaust valve. Thanks HAMB.
    clunker, wicarnut and belyea_david like this.
  11. praisethelowered
    Joined: Aug 14, 2003
    Posts: 1,103


    It went between 0 and 90...I couldn't get it to read the same twice
    All the rest of the cylinders are at 145-150

    I am hoping this is good news and that I just need a valve job
  12. Bill Rinaldi
    Joined: Mar 23, 2006
    Posts: 1,863

    Bill Rinaldi

    Had a great running early 350 in my truck, after I ran it for 20,000 or so miles, I had same problem as you. Pulled heads and found they were "lead heads", not designed for non ethyl gasoline. Got a set of later year 350 heads, with non ethyl compatable valves and valve seats, end of problem.
  13. Jmountainjr
    Joined: Dec 29, 2006
    Posts: 1,377


    If the heads are '63 vintage and never updated, I'd pull both heads and have hardened valve seats installed on the exhaust valves. As stated above, the no lead gas kills the older heads. Or update the heads with newer if not concerned with keeping the engine period correct.
  14. 4wd1936
    Joined: Mar 16, 2009
    Posts: 1,002

    from NY

    Any chance it could be a broken valve spring?
  15. Engine man
    Joined: Jan 30, 2011
    Posts: 3,480

    Engine man
    from Wisconsin

    Do a used car lot valve job. Take off the valve cover, rocker arm and valve spring on the exhaust valve using an air hold. Use a power drill to spin the exhaust valve. You can reduce the air pressure until it the valve turns and allows you to open the exhaust valve. Having air pressure in the cylinder forces any debris to go into the exhaust system. This can help seat a valve with light rust.

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