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Technical Too low of vacumn to modulator valve ?

Discussion in 'The Hokey Ass Message Board' started by junkyardjeff, Nov 29, 2020.

  1. Mimilan
    Joined: Jun 13, 2019
    Posts: 1,250

    Mimilan
    Member

    How much distance between your modulator vacuum source and your PCV vacuum source.
    The PCV will flow volume, whereas the Modulator only needs Vacuuum [negative pressure]

    The Modulator source should be on a manifold runner . If you did it the other way round and hooked up the PCV to a manifold runner, it would be enough to lean out some cylinders [a controlled vacuum leak]

    Your vacuum could be great at idle, but when driving down the road the PCV could be reducing the vacuum to the modulator
     
  2. junkyardjeff
    Joined: Jul 23, 2005
    Posts: 8,694

    junkyardjeff
    Member

    PCV is coming from the big port on the Holley 4 bbl and the modulator is hooked to the port on the carb spacer. When I adder power brakes the only port left I think is on a intake runner so it looks like I will have to re plumb the booster and modulator.
     
  3. Budget36
    Joined: Nov 29, 2014
    Posts: 15,195

    Budget36
    Member


    Before the rebuild, try cleaning up the valve body as was suggested before...little time, little fluid, a gasket (maybe a mess) but it's what I would do with the advice offered before.
     
  4. Mimilan
    Joined: Jun 13, 2019
    Posts: 1,250

    Mimilan
    Member

    The port on the carb usually plumbs into the centre so that all 4 throats are equal, and you don't have a leaning out on 1 side situation.
    Your modulator is effectively plumbed underneath the PVC so it would see a vacuum drop.

    Try a temporary hook up to where the brake booster goes. [even if it means disconnecting the booster it to see if it works]

    On a '66 mustang C4 the modulator is connected to a manifold runner [behind a carb] tee'd with the booster
    Both the booster and the modulator require vacuum, not flow .

    I hope it is as simple as this for you!
     
  5. Charlietruck62
    Joined: Apr 2, 2019
    Posts: 61

    Charlietruck62

    Do this. All Ford modulators are hooked to a manifold runner vacuum source.
     
  6. junkyardjeff
    Joined: Jul 23, 2005
    Posts: 8,694

    junkyardjeff
    Member

    PCV and booster are hooked to where they get vacumn from all 8 and the modulator is on a runner,I changed this before I fired it up and the initial third gear shift still did not happen until around 45 then it started shifting normally. Put some of that Trans X stuff in it and let it go through all the gears and will see how it does tomorrow. Going to adjust the bands before firing it up tomorrow,if I can get it to start acting normal I start on getting the Y block and COM ready to install. I really do not want to rebuild the C-4 and then shortly after get the 62 COM gone through.
     

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