Compression release

Discussion in 'Airheads' started by Plaka, Feb 7, 2019.

  1. Plaka

    Plaka Brevis illi vita est

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    I'm trying to figure out how to do a compression release on a dual plugged head. The lower plug is small/short reach.

    I want to be able to crank the engine with the ignition off against no compression, but there doesn't seem to be room in the head for another valve. So maybe some way to hold the existing valves open?

    On the other hand with the throttle closed there shouldn't be much air in there so maybe I'm worrying about nothing? Sort of like doing a compression test with the throttle closed.
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  2. robtg

    robtg Long timer

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    Why?
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  3. greentreephrog

    greentreephrog Been here awhile

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    If you really need to do it, have a look at the valve cover and associated parts of a 1945-55 AMC 500 single. Or probably any such bike of that era.
    #3
  4. Plaka

    Plaka Brevis illi vita est

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    Um, yeah. Got one of those right here behind the Studebaker somewhere....

    Actually in writing the post I gave myself a clue.I should do a throttle closed compression check and see how much I have. In theory after the first exhaust stroke the only air in the cylinder is what sneaks past the rings on the intake stroke + what little gets under the bing butterfly.

    I want an aircraft style preoiler. I park it with the carbs dry so if I crank it until the oil pressure gauge reads x then turn on the gas and the ignition I should be set.

    I have the scarf.
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  5. aptbldr

    aptbldr easy rider

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    I'm imagining unwarranted complex-isity.
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  6. Plaka

    Plaka Brevis illi vita est

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    There are medications for that.
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  7. bpeckm

    bpeckm Grin! Supporter

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    I do that every spring on first startup, just by pulling the plug (you would only need to pull the top plug) and laying it on the cylinder with the ht lead connected, then crank until the oil light goes out. The fuel has not yet been turned on, so bowls are dry.

    No reason you couldn't do it with plugs in, just using more battery oomph that needs to be replaced (run it or charge it!).
    #7
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  8. Pokie

    Pokie Just plain Pokie.

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    If you want to crank the engine with the ignition off, just pull one plug from each head. It's quick, it's easy. If you desire to use the electric starter, the ignition will need to be on, unless you also have a switch to ground the points.
    #8
  9. Plaka

    Plaka Brevis illi vita est

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    The electrics aren't an issue. If I want to spin the starter and as long as the bike isn't running it will spin, just push the button. My bike my wiring. (I don't use ignition switches anyway exactly). I've been parking with the carbs dry for years so nothing extra to do there. Already bought a high output starter to reduce battery drain. just need to figure out the compression I'll be workingh against.

    I've been thinking about this for years. Aircraft use pre oilers for a reason, the TBO and huge cost of engine wear. Now I'm contemplating running a worn bottom end with a powerful top end and all is apart anyway so it seemed like time to move on the project. I really don't want to go the aircraft route with a separate motor, pump and plumbing, much as I would like some jets directed at the cam (and may put some in anyway. plenty of room in the pan to spray up.)
    #9
  10. photorider

    photorider Been here awhile

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    Is this coming from the same person who thought the handlebar choke lever was too complicated?
    Seems pointless to me on an Airhead engine but your bike, your time.
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  11. thetubespoke

    thetubespoke Been here awhile

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    If you don't mind going single plugged, maybe a solenoid activated pressure release in the lower plugs. Might not be too hard to build.

    If not that, maybe something sticking out of the valve cover like a screw to pull the exhaust valve open on either side. Of course need to be careful not to have the valve open too far when cranking if it's an interference motor (I'm guessing these are but not sure).
    #11
  12. Piggdekk

    Piggdekk Been here awhile

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    hell they are :grim
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  13. Plaka

    Plaka Brevis illi vita est

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    You never understood that; you don't understand this.
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  14. Plaka

    Plaka Brevis illi vita est

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    "pull" the exhaust valve open? I have two other lever cam mechanisms on the build and they work well but none have to hold oil. and then I would have to get around whatever it is when I adjust valves, and make sure it cannot be employed with the engine running. Dotting wedges on the rockers, etc.....I have been down those paths.
    #14
  15. Plaka

    Plaka Brevis illi vita est

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    If we had some ham we could have ham ' eggs if we had some eggs.
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  16. photorider

    photorider Been here awhile

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    You’re right about that! Carry on...
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  17. James.A

    James.A ILL Airmarshal

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    Did you ever take a compression reading with the carburetors in place? Trust me, air does get in to the cylinder. The best way to defeat compression is to pull a spark plug and make sure to ground the plug against the base of the cylinder.
    #17
  18. Plaka

    Plaka Brevis illi vita est

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  19. dmftoy1

    dmftoy1 Long timer

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    One thing to look into that might be a lot simpler would be an oil pressure accumulater. In a past life we used to mount them in jeeps so if you got a bad angle or rolled it they would automatically feed pressurized oil into the system. No reason you couldn’t have a simple valve to trap pressurized oil in the accumulator and then release it pre-start
    #19
  20. bajaburro

    bajaburro Ancient Adventurer

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    I wouldn't worry to much about the life of that engine,sadly it will out live you.Just say'n.
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