Sticking vacuum piston plate

Fuel and Air Systems

  1. Ken71Twister

    Ken71Twister FABO Gold Member FABO Gold Member

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    I’ll add a photo to help clarify... I just used a carb kit to rebuild a 2 barrel Carter carb for my 1971 318 Duster. I cleaned everything a well as I could using a carb cleaner, rags, etc. I noticed that the vacuum piston plate assembly wasn’t moving freely when re-assembled. It moved fine before the retaining screw was added - but intermittently sticks with the retaining screw added. I installed the carb and wasn’t too surprised when it didn’t idle well. Couldn’t make any improvement with idle mixture screws. The car starts great - but idle is not good. It runs ok at higher rpm and takes gas well when throttle is opened.

    Any suggestions on how to get this piston to move properly? E5082DF7-1492-4ECD-B5B7-1E78262BF73D.jpeg
     
  2. Ken71Twister

    Ken71Twister FABO Gold Member FABO Gold Member

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    For what it’s worth... the car idled perfectly before the carb rebuild... so I’m inclined to believe the cause is in the carb. I did the rebuild because the accelerator pump wasn’t working and it was hard to start when cold.
     
  3. Dana67Dart

    Dana67Dart Like a fine wine, only getting better with age! FABO Gold Member

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    Just a thought.

    Remove the metering rods and try the movement. If it moves freely the rods might be bent, mis installed, or the wrong ones for the Jets.
     
  4. Ken71Twister

    Ken71Twister FABO Gold Member FABO Gold Member

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    It moved freely with rods installed but before the retaining screw was installed. Gut tells me that the shoulder of the stud is dragging against the guide sleeve in collar on the top of the piston. (The stud that I refer to is in the top-center of my photo). The pin/jet combo were there before I did the rebuild - so they should be the right size. The parts that don’t move freely appear to be shiny clean..but I’m wondering if I need to move a little metal to increase clearance between the two parts that are binding. In the photo... the piston is stuck in the down position. I’m not sure what forces move this thing up and down when things are buttoned up and the car is running - but I’m guessing vacuum from under the piston.
     
  5. Ken71Twister

    Ken71Twister FABO Gold Member FABO Gold Member

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    Looks to me that these two parts cannot be properly aligned - thus the reason the piston isn’t free to move when the screw is installed

    7EEAAF99-23EE-4270-8BB8-8AB4887359C0.jpeg

    B056DA1C-197F-4099-A8D8-F5A5C79F1B4E.jpeg
     
  6. AJ/FormS

    AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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    That is odd; But now you know what to do.

    Nice cleaning job.
     
  7. Ken71Twister

    Ken71Twister FABO Gold Member FABO Gold Member

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    Quick update.... I pulled apart another Carter carb that I had stashed away and used the vacuum piston from that carb. It fits fine and all seems to move well. I noticed that the older one (that fits) has a Carter stamp in it and the other one didn’t. Apparently that piston hasn’t worked since I installed that rebuilt carb several years ago. My money says that the piston was stuck in the down position which allowed the car to idle well but probably suffered during acceleration. I look forward to my next drive. Right now I’m waiting for shipment of a new fuel line (pump to carb). I noticed that the old one had rust inside and was sending small bits of debris into my cleaned carb bowl. I’ve had a heck of a time finding a good match on a new fuel line but finally ordered one last night. I may get impatient and clean the old fuel line for temporary use.
     
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