Deceleration valve leaking?

Mopar Exhaust Systems

  1. pishta

    pishta I know I'm right....

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    or (Ford) Gulp valve or anti-backfire valve, AMC Decel-valve.....used on 70's era cars to prevent backfiring when letting off the gas. Jeep used them, cant remember a Mopar application. Question: is the vacuum pod supposed to hold a vacuum? My little Mazda truck wont idle and I found this part (I bought it NOS 7 years ago and put maybe 1000 miles on it) wont hold a vacuum so I assume this is the vacuum leak or part of it. I guess its supposed to open a large air to exhaust manifold feed upon deceleration to dilute the rich A/F when you snap the carb shut. This part is N/A today so I got to figure out what I can do. Seems the 3/4" passage goes right from the air cleaner through the valve right into the intake manifold where the PCV valve is on my intake. So I'm guessing there should be a leak free seal at the outlet to the intake manifold as well as the vacuum line to the control diaphragm. Both seem to be leaking slightly although the big one is leaking less than the small 1/8 vacuum control line. Upon higher than idle vacuum (25" decel or right off the gas) the valve should open momentarily and dilute the intake with air preventing an explosive ratio. Vacuum drops, the valve closes and it stays closed until it decels again.
     
  2. Rmoore

    Rmoore Well-Known Member

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    Have no clue what I just read !! Wow !
     
  3. Rmoore

    Rmoore Well-Known Member

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    The only one way valves on Fords that I know of were for the thermactor system !! But they were never for the engine backfiring !! If the engine is backfiring it will need be fixed tho !!
     
  4. pishta

    pishta I know I'm right....

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    Engine has a massive vacuum leak, and I believe its this POS valve do-hickey. Not 100% on its working dynamics but I assume a leak anywhere in this is not good. Literally a 3/4" feed between the air cleaner base and the intake manifold with this valve in the middle controlled by a 25" spike of manifold vacuum . Strange thing is that it is fairly new (installed 5 years ago and under 1000 miles on it, but it was old stock, 1976! Vacuum diaphragm is 44 years new.
     
  5. Rmoore

    Rmoore Well-Known Member

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    lol ! I hear ya brother !! All manufactures had some funky stuff going on back in the day ! I’ve been a Parts Manager at a Ford dealer for the past 30 yrs.. all at the same dealer ! For life of me I don’t remember anything like that on Fords that wasn’t part of the Thermactor system !! But hey !! I don’t care how along you been doing something you can still learn something new !! Good luck to you brother !!
     
  6. Murray

    Murray FABO Gold Member FABO Gold Member

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    Maybe you are thinking of the EGR valve which is a massive air leak?
     
  7. krazykuda

    krazykuda Well-Known Member FABO Gold Member How-To Section Editor

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    Could you use a check valve to keep it from leaking???
     
  8. Syleng1

    Syleng1 Karma is real and Life is short... FABO Gold Member

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    How about a picture? Some of us are old enough to have worked on carbs during the early 80’s with feedback carbs and solenoids galore. It may help getting our memories jogged as we drink our Metamucil for the day.

    Back then, manufactures were scrambling to meet the emissions new guidelines and were throwing everything but the kitchen sink at engines to make them compliant. 2 gas tail pipe testing and crazy fuel tank additives and Native American dance rituals to get your car to pass emissions. Crazy times.
     
  9. Garrett Ellison

    Garrett Ellison Amateur driver on public roadway, do not imitate.. FABO Gold Member

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    Going by my experience of Nissan's pickups (Z-20, Z-22, & Z-24). They used a similar setup but it only let air in from the air cleaner into the intake. No exhaust connection. Did you mean to say intake instead of exhaust? But you are correct on the operation, spring pressure holds the valve shut and high vacuum during decel opens it. The easiest way to check it is to cap your vacuum fitting at the carb and plug the bypass air inlet from the air filter or pull the valve and plug the intake opening to see if it idles. Hope this helps!
     
  10. pishta

    pishta I know I'm right....

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    @Garrett Ellison , the EGR goes to the same location on the intake, a large threaded boss that has the 2 fittings: EGR and gulp valve inlet so no exhaust is present. Datsun used the same contraption but I think the Datsun had a temperature sense fitting on their's too. It was weird: It was idling after I did a fuel pump and filter...and then it just died. Thought I had run out of gas. Fuel sight was still full...Hard to start after words and had to feather the throttle to keep it running. Classic vacuum leak and a big one. cleaned the carb thinking the air bleeds were plugged up or the accelerator pump was shot but the carb is NOS also 5 years, 1000 miles. This motor has the pulse injection, just a big exhaust manifold inlet attached to the air cleaner through a giant (12) one way valve. Loud as hell with the air cleaner off. Ill take some pics tonight. 6 is the suspect....could be a F'd PCV (9).
    90574v08L.gif
     
    Last edited: Jul 20, 2020
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    • AJ/FormS

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

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      Looks like a CAT protection device to me, to promote end-gas burning in the manifold, prior to entering the CAT.
      No CAT/ no problem,defeat it.
       
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      • pishta

        pishta I know I'm right....

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        Oof, i still got a CAT and have to have it hooked up per CA smog stranglegold. I suppose I could defeat it somehow, plug the inlet...will investigate further tonight. Should be able to just plug the air cleaner feed and then it can leak all it wants as it wont have any fresh air supply. Vacuum sense line is easily pinched off.
         
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        • AJ/FormS

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

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          If you defeat it with the CAT still in there, Have you given a thought as to how the Cat will deal with the resulting rich-on-decel, mixture?
           
        • pishta

          pishta I know I'm right....

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          I think one outweighs another: running or protecting the CAT? Hadn't thought of the consequences but what's a little heat to an old CAT or is the possible backfire a more pressing issue? I juust want this thing to friggin run again. And if i need a $30 valve off an 80 Fairmont, ill get one. The original Mazda version was >$400 5 years ago. Just had a different layout, 90 degree vs Fairmont model that is inline. Not an issue.
           
        • AJ/FormS

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

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          I have only recently acquired a car with a CAT, so IDK the consequences of what you are contemplating. I thought perhaps you knew.
          Riddle me this;
          We know the CAT has to be there, but does testing know it's working,lol.
           
        • pishta

          pishta I know I'm right....

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          The CAT reduces hydrocarbons and 3 ways (1981) reduce NOx and actually need to run a little rich to work (!) So a little rich wont hurt it unless it goes into meltdown as in a rich mix at all times. Were dealing with just very short periods of rich mix, as in less than a second an event. I remember my sis's carbed Toyota Corolla always 'coughing' on lifting the gas when she shifted as well as most of the manual trans Toyotas of the time. Maybe all their decel valves were wasted or they didnt use them. I know Datsun used them and i got a few hits on their workings on Datsun sites.
           
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          • pishta

            pishta I know I'm right....

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            UPDATE>>>

            I think I found the issue...bad F'n gas!

            Removed everything associated with manifold vacuum: power brake, EGR, distributor, evap shutter, PCV, gulp valve...plugged them all and the thing ran the same, crappy. WTF, had kid keep the revs to 1200 and sprayed all the gaskets, no change. Finally bit the bullet and increased the idle speed 2 turns to 1200, got it to fast idle without pedal pressure and started shooting the carb with carb cleaner. Sputtered and cleared, again...sputtered and cleared......finally after about 3 minutes of crappy fast idle, the thing just settled in and started to idle smooth! Hey there....turned the idle down 2 turns to its original base idle and voila. Smooth 700 RPM idle. Started to add vacuum acccessories:
            1. power brakes, no change.
            2. distributor vacuum (changed for the better!)
            3. PCV (clean now) no change!
            4. Evap shutter...no change
            5. Anti gulp valve---no change!

            Verdict: The carb low speed air bleeds (?) varnished over due to non use and were plugged FUBAR. After carb cleaner opened them up, everything fell into place. Only thing I can think of as this happened once before on my original worn out carb. Sure felt like a vacuum leak as I had to feather the throttle just to keep it running like a leak would act. Im thinking the entire idle circuit was plugged up with this new shitty gas that doesn't age well. 2 turns took me out of idle circuit. Damn lawn mower acts the same: must be started every week or the bowl gets jello formed in it and main jet clogs.
             
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