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Carb piston question

Discussion in 'XJ Technical Chat' started by gitbox, Jul 12, 2009.

  1. gitbox

    gitbox Member

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    While doing another carb cleaning and I noticed this: The piston on the left in the pic with the fourth hole does not match the other three pistons. They all have a plug in that hole like the one on the right. Does this make any difference? Should I get one that matches? Am I worrying for nothing? Will these questions ever stop?

    As always, thanks for the help.
     

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    gitbox, Jul 12, 2009
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  2. ktcubed

    ktcubed Member

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    Don't know the answer to your actual question, but looking at the white plastic retaining ring, the one with four holes looks newer than the one with three holes.
     
    ktcubed, Jul 12, 2009
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  3. cturek

    cturek Member

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    Good question. Mine have the two smaller holes, but I've never noticed a filled third hole. (I have hitachi's) I have been wondering about any of the holes. Wouldn't they "short circuit" the pressure differential between above and below the diaphram? Just like a tear in the diaphram?
     
    cturek, Jul 12, 2009
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  4. RickCoMatic

    RickCoMatic Well-Known Member

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    That missing Plug needs to be replaced or that Diaphragm Assy swapped.

    Either the Plug fell out ... (Unlikely)
    Or, got driven-out ... (Likely) ... by somebody who set-up the Carbs that Piston came from with a Stage-1 ~ 3 Kit.

    Compare the Needle Valve that is on that Piston to the other three.
    You might find that the Needle isn't the same Diameter or Length.

    Cut a SMALL Circular PATCH from a short-end piece of Shim Stock
    "Sweat" the Patch on over that open hole.
    Sand the Solder-Job down nice and flat and shiny.
     
    RickCoMatic, Jul 12, 2009
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  5. MiCarl

    MiCarl Active Member

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    Actually - the holes allow the pressure differential on the diaphragm.

    As the air velocity under the piston increases pressure drops. Those holes transmit the pressure drop to the top of the diaphragm. The bottom of the diaphragm is vented to the atmosphere. Atmospheric pressure pushes the diaphragm up, lifting the piston.

    The piston rising opens the throat of the carburetor, reducing the air speed. When everything comes into balance the piston stops moving. This type of carburetor is called "constant velocity" because the speed of the air under the piston doesn't change (much).

    When the piston moves up it also lifts the jet needle allowing more fuel for the larger amount of air moving through the carburetor.
     
    MiCarl, Jul 12, 2009
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  6. cturek

    cturek Member

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    Thanks Carl. That clears up something I have been wondering about for quite some time.
     
    cturek, Jul 13, 2009
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