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Plenum vacuum check flapper

Discussion in '206/246' started by jselevan, Oct 12, 2025 at 1:00 PM.

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  1. jselevan

    jselevan Formula 3

    Nov 2, 2003
    1,881
    #1 jselevan, Oct 12, 2025 at 1:00 PM
    Any suggestions on an alternative to the plastic flapper that sits in the cage atop the crankcase vacuum outlet inside the plenum? I have been using a round piece of plastic that I for years. But I cannot rid my 02900 of popping on trailing or idle throttle. Have cleaned all idle jets and low-speed jets, checked for air leaks, etc. Adjusted the idle jets meticulously (over and over and over....). Thinking it might be air leaking by my MacGyver flapper that I put in place before most of you were born (1978 or thereabouts).

    Thoughts? I'm trying to 2-part epoxy a piece of round rubber onto the round plastic to form a seal along the edges where it sits on the plenum manifold. Not sure that the rubber will safely stay on the plastic using epoxy.

    Thanks.
    Jim S.
     
  2. mar3kl

    mar3kl Formula Junior

    Nov 17, 2011
    474
    Silicon Valley
    Full Name:
    Mark
    #2 mar3kl, Oct 12, 2025 at 4:04 PM
    I bought a sheet of phenol and hand turned a disk which has worked quite well over the past ten years.
     
  3. jselevan

    jselevan Formula 3

    Nov 2, 2003
    1,881
    #3 jselevan, Oct 12, 2025 at 4:06 PM
    Thanks for sharing your experience. Is phenol hard? Or flexible?

    Jim
     
  4. mar3kl

    mar3kl Formula Junior

    Nov 17, 2011
    474
    Silicon Valley
    Full Name:
    Mark
    #4 mar3kl, Oct 12, 2025 at 8:03 PM
    At the size of this disk, it’s not going to flex, and it’s lightweight. I think thickness was something like 1mm, maybe a bit less. If I remember right, it’s what was originally used on the car. Cheap and easy to find online.
     
  5. mar3kl

    mar3kl Formula Junior

    Nov 17, 2011
    474
    Silicon Valley
    Full Name:
    Mark
    #5 mar3kl, Oct 12, 2025 at 8:07 PM
    That cage assembly works with a pair of narrow flexible tubes connected to a pipe on the outside of the plenum, and those are frequently missing, so that may be part of your popping problem. It’s been years since I looked at that part of the system, but I remember bending my hands into awkward shapes trying to get all that hooked up.
     
  6. TonyL

    TonyL F1 Rookie

    Sep 27, 2007
    4,248
    Norfolk - UK
    Full Name:
    Tony
    #6 TonyL, Oct 13, 2025 at 2:45 AM
    when you say popping, is that through the exhaust on deceleration or snapping / spitting from the carbs?

    I doubt the flapper valve is causing your issue.

    Popping through the carb is a symptom of lean condition, through the exhaust is possibly and air leak in the exhaust system or intake leak. Its important to know which problem you have in order to narrow down the issue.
     

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