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Please could somebody explain the purpose of this plastic plug on the bottom of my SU HS4 carburettor piston? The lip of the plug protrudes very slightly from the bottom of the piston (shown bottom left) and prevents the piston from contacting the carb's bridge when in the fully down position, leaving a minute gap instead of a flush metal-metal connection. Is this intentional?

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    Probably keeps it from making noise when closing throttle. Its there for a reason.
    – Moab
    Apr 25, 2020 at 15:28
  • @Moab I wondered also whether it was to leave a minuscule gap on a closed throttle to allow air to reach the jet and through to the engine. Otherwise, what is causing the fuel to be drawn from the jet? It just seems curious I cannot find any information on this and Haynes seems to suggest that it should be a flush metal-metal connection.
    – user1083734
    Apr 25, 2020 at 19:00
  • Having spent too much of my life trying to persuade SU carbs to work properly when they were "state of the art", I don't remember ever seeing that feature.
    – alephzero
    Apr 25, 2020 at 21:38
  • @alephzero have added a photo for clarity
    – user1083734
    Apr 29, 2020 at 22:23

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