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XJ900 31A BS35 Mikuni Vacuum Slide (piston) vacuum hole size?

Discussion in 'XJ Technical Chat' started by cds1984, Jan 10, 2026 at 2:28 AM.

  1. cds1984

    cds1984 Well-Known Member

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    I Searched and searched but...

    My slide vacuum holes have been drilled out to 3.5mm(ish, it might be imperial), I'm looking for the original size of the vacuum hole in the slide for the Mikuni BS35.

    Thanks in advance.
     
    cds1984, Jan 10, 2026 at 2:28 AM
    #1
  2. cds1984

    cds1984 Well-Known Member

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    Found an undrilled slide.
    3/32" barely squeezes in and 2.5mm is too large.
     
    Last edited: Jan 10, 2026 at 11:08 PM
    cds1984, Jan 10, 2026 at 11:00 PM
    #2
  3. Simmy

    Simmy Well-Known Member

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    Now you just need to drill 3 more slides smaller
    You could epoxy them then drill to any size.
    Why do you want to go to the OEM hole?
    I’ve drilled out a few banks with understanding it just quickens the slide action.
     
    Simmy, Jan 11, 2026 at 4:12 PM
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  4. cds1984

    cds1984 Well-Known Member

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    Just did the 58l pistons in 31a cylinders and have a stumble at low end now.
    Drop the needles and it's gone but the mid to high range is teeth grinding, whereas it is good at original position.
    Epoxy is done. Waiting...
     
    cds1984, Jan 11, 2026 at 5:51 PM
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  5. Minimutly

    Minimutly Well-Known Member

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    Indeed it does quicken the slide action, but this reduces the snap enrichment fuelling, which usually gives you a weak stumble, especially if you overdo it.
     
    Minimutly, Jan 12, 2026 at 3:24 AM
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  6. cds1984

    cds1984 Well-Known Member

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    I figure I have full compression now so the flow is greater through the venturi. Previous I was down to 90 on the best cylinder and the drill out worked a charm.
    Never satisfied obviously.
     
    cds1984, Jan 12, 2026 at 5:10 AM
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  7. Minimutly

    Minimutly Well-Known Member

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    How does compression change the flow through the venturi then?
    Answer is, it doesn't. Flow through the venturi is a product of cylinder volume, piston velocity, cylinder head efficiency, and the big one, how much the throttle is open.
     
    Minimutly, Jan 12, 2026 at 6:23 PM
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  8. cds1984

    cds1984 Well-Known Member

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    Good point. Something is different though. So going back to standard.
     
    cds1984, Jan 12, 2026 at 6:47 PM
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