afb 4131s surge at wide open throttle

Discussion in 'Fuel and Air Systems' started by stroker402, Sep 2, 2018.

  1. stroker402

    stroker402 1968 dart GTS convertible

    Messages:
    813
    Likes Received:
    300
    Joined:
    Jan 30, 2006
    Location:
    milwaukee wi.
    Local Time:
    9:19 AM
    I have a carter afb 4131s (575-600 cfms) specs show , that I have gone thru and rebuilt . I have a full throttle surge that comes on around 5000 rpm when the secondaries are opening up. (just when you are getting into the secondaries) . everything is fine on the primary side. no issues there. also no issues with timing , distributor or any other electrical components . I just recently took off a holley 650 dp that worked just fine at full throttle. thinking that dp was just a little too much . I probally had this carb off the car and gone thru it dozen times but with no luck getting that surge to go away. here are what is in it now . step rods 16-404 brown springs and rod tall covers. .. tall jets in pri 120 - 352 have no clue what this is in orifice size. can't be 52. rear jet 392 . I went thru this carb ,soaked it, blew every orifice out a dozen times , set the floats to stock specs, new needle and seats. everything appears good there. squirters are working fine . this particlular carb does not have any secondary weights or butterflies in it all oem components. all gaskets are aligned up with the correct holes every where.
    since the floats are set to stock measurements , do you think that going up in the secondary jet may cure this ? or maybe setting the floats different ? I just don't know where to go from here.The engine is a fresh rebuilt 273 commando..... one step up in cam and headers new mech fuel pump . That's it.
    where should I look next?
     
  2. Murray

    Murray FABO Gold Member FABO Gold Member

    Messages:
    508
    Likes Received:
    207
    Joined:
    Apr 19, 2010
    Location:
    San Jose, California
    Local Time:
    7:19 AM
    A surge indicates a lean condition. Usually the causes are more simple like low float level, plugged fuel filter, fuel pump, non-vented gas cap, etc. I think your case is different. The main jets may be too small. I would be curious about the main size on the old carb versus the new one.
     
    • Like Like x 2
    • Agree Agree x 1
    • stroker402

      stroker402 1968 dart GTS convertible

      Messages:
      813
      Likes Received:
      300
      Joined:
      Jan 30, 2006
      Location:
      milwaukee wi.
      Local Time:
      9:19 AM
      just spent some more time researching this issue appears someone has been in this carb before me switching things . it is suppose to have a #89 tall jet not a #52 jet . also the metering rods been switched too . I'll put the correct rod and jet in and see if that helps.
       
    • Murray

      Murray FABO Gold Member FABO Gold Member

      Messages:
      508
      Likes Received:
      207
      Joined:
      Apr 19, 2010
      Location:
      San Jose, California
      Local Time:
      7:19 AM
      That sounds right- good luck.
       
      • Like Like x 1
      • Murray

        Murray FABO Gold Member FABO Gold Member

        Messages:
        508
        Likes Received:
        207
        Joined:
        Apr 19, 2010
        Location:
        San Jose, California
        Local Time:
        7:19 AM
        That sounds right- good luck.
         
      • Murray

        Murray FABO Gold Member FABO Gold Member

        Messages:
        508
        Likes Received:
        207
        Joined:
        Apr 19, 2010
        Location:
        San Jose, California
        Local Time:
        7:19 AM
        That sounds right- good luck.
         
        • Thanks! Thanks! x 1
        • stroker402

          stroker402 1968 dart GTS convertible

          Messages:
          813
          Likes Received:
          300
          Joined:
          Jan 30, 2006
          Location:
          milwaukee wi.
          Local Time:
          9:19 AM
          We I did some more digging into this 4131s carter afb and found out that the 120 - 352 tall jet has the same orifice as the .089 tall jet . carter used different numbers over the years with same orifice size. appears that someone had this carb running really fat . so I am leaning it out some with a 74 sec jet from 92 sec jets and leaner step rods. It may be that the issue it had is maybe it was running too rich. I'll see when I get it running again. it's just a little 273 commando engine .
           
        • stroker402

          stroker402 1968 dart GTS convertible

          Messages:
          813
          Likes Received:
          300
          Joined:
          Jan 30, 2006
          Location:
          milwaukee wi.
          Local Time:
          9:19 AM
          took it out for a ride and it is still breaking up when secondaries start to open . It breaks up even more with the air cleaner on . so......... it breaks up with a 74 rear sec jet now even with the previous sec jet 92 . it's been at each extreme high and low ....... lean and rich condition and still breaks up. running out of things to try.
          One thing good is it flies like a bat out of hell on the primaries though.
          going to pull the vac advance hose off and try next..
           
        • Mike 340

          Mike 340 Well-Known Member

          Messages:
          126
          Likes Received:
          33
          Joined:
          Jun 21, 2016
          Location:
          Canada
          Local Time:
          10:19 AM
          I believe that you need the secondary weighted butterfly valve, I have never seen one before that does not have them .
          That would explain your lean condition .
          Just a thought?
          Mike
           
        • Murray

          Murray FABO Gold Member FABO Gold Member

          Messages:
          508
          Likes Received:
          207
          Joined:
          Apr 19, 2010
          Location:
          San Jose, California
          Local Time:
          7:19 AM
          A couple of things to think about. The previous Holley that worked OK- how do the main jets compare to the AFB? Could the surge be in the transition from primary to secondary? Maybe the primary is too lean. Just a thought.
           
        • stroker402

          stroker402 1968 dart GTS convertible

          Messages:
          813
          Likes Received:
          300
          Joined:
          Jan 30, 2006
          Location:
          milwaukee wi.
          Local Time:
          9:19 AM
          It's a carter afb 4131s no signs of any weights in the secondaries.
           
        • stroker402

          stroker402 1968 dart GTS convertible

          Messages:
          813
          Likes Received:
          300
          Joined:
          Jan 30, 2006
          Location:
          milwaukee wi.
          Local Time:
          9:19 AM
          Previous holley dp 4777 has 67 pri and 76 sec jets. anyone know what these jets would be equivlent to my carter afb 4131s with 89 pri and 82 sec?
           
        • stroker402

          stroker402 1968 dart GTS convertible

          Messages:
          813
          Likes Received:
          300
          Joined:
          Jan 30, 2006
          Location:
          milwaukee wi.
          Local Time:
          9:19 AM
          I took it for a ride with the vac line off and plugged . still surging wot .
           
        • stroker402

          stroker402 1968 dart GTS convertible

          Messages:
          813
          Likes Received:
          300
          Joined:
          Jan 30, 2006
          Location:
          milwaukee wi.
          Local Time:
          9:19 AM
          Jetted the secondaries up to a 98 jet . seemed to be a bit better only if you ease into the throttle to 5k rpms. If you punch it wot at a roll it just won't clear itself out and surge .
           
        • Mike 340

          Mike 340 Well-Known Member

          Messages:
          126
          Likes Received:
          33
          Joined:
          Jun 21, 2016
          Location:
          Canada
          Local Time:
          10:19 AM
          Is it possible that it is missing?
          I have worked on that type of carb, the secondary air valve just kinda "sits in there "
          Seems odd to me?
          Not like I have been wrong before !
          Mike
           
        • stroker402

          stroker402 1968 dart GTS convertible

          Messages:
          813
          Likes Received:
          300
          Joined:
          Jan 30, 2006
          Location:
          milwaukee wi.
          Local Time:
          9:19 AM
          disconnected the tach wire and bumped up the rev limiter to 9k so it wouldn't hit it a 5500 rpms . still surging . no improvement .
           
        • Mike 340

          Mike 340 Well-Known Member

          Messages:
          126
          Likes Received:
          33
          Joined:
          Jun 21, 2016
          Location:
          Canada
          Local Time:
          10:19 AM
          Can you post a picture of the top of the carb ?
           
        • Mike 340

          Mike 340 Well-Known Member

          Messages:
          126
          Likes Received:
          33
          Joined:
          Jun 21, 2016
          Location:
          Canada
          Local Time:
          10:19 AM
          Looking down.
          Mike
           
        • stroker402

          stroker402 1968 dart GTS convertible

          Messages:
          813
          Likes Received:
          300
          Joined:
          Jan 30, 2006
          Location:
          milwaukee wi.
          Local Time:
          9:19 AM
          Mike............on the insides of the 4131s afb , there is no sign of any kind of mounting for the weights in it . All looks stock in the insides. just can't see any where the weights would sit in there. they have to have some kind of slot to rest/rotate on . there is nothing like that in there.
           
        • stroker402

          stroker402 1968 dart GTS convertible

          Messages:
          813
          Likes Received:
          300
          Joined:
          Jan 30, 2006
          Location:
          milwaukee wi.
          Local Time:
          9:19 AM
          my digital camera came up with a "memory card error " . I don't know how to turn it off/ unlock it . otherwise I would shoot you guys some pics of the insides.
           
        • stroker402

          stroker402 1968 dart GTS convertible

          Messages:
          813
          Likes Received:
          300
          Joined:
          Jan 30, 2006
          Location:
          milwaukee wi.
          Local Time:
          9:19 AM
        • stroker402

          stroker402 1968 dart GTS convertible

          Messages:
          813
          Likes Received:
          300
          Joined:
          Jan 30, 2006
          Location:
          milwaukee wi.
          Local Time:
          9:19 AM
          Bill .......here is a inside pic of the 4131s ..... the throttle blades are all closed . no weights in rear .

          IMG_1962 (Small).JPG
           
          Last edited: Sep 4, 2018
        • Murray

          Murray FABO Gold Member FABO Gold Member

          Messages:
          508
          Likes Received:
          207
          Joined:
          Apr 19, 2010
          Location:
          San Jose, California
          Local Time:
          7:19 AM
          Some older AFB's had a counterweighted secondary air valve above the secondary butterflies. I think this is what Mike is referring to. It would open as the air velocity would increase above the butterflies. I assume your carb is a later design. Good luck on the camera.
           
          • Thanks! Thanks! x 1
          • stroker402

            stroker402 1968 dart GTS convertible

            Messages:
            813
            Likes Received:
            300
            Joined:
            Jan 30, 2006
            Location:
            milwaukee wi.
            Local Time:
            9:19 AM
            Thanks Murray ... I'm sure that is what Mike was referring to. He knows his stuff. Sorry Mike !
             
          • stroker402

            stroker402 1968 dart GTS convertible

            Messages:
            813
            Likes Received:
            300
            Joined:
            Jan 30, 2006
            Location:
            milwaukee wi.
            Local Time:
            9:19 AM
            update !
            Well I finally gave up on this 4131s carb.. I purchased another afb 4294s from a 67 commando . installed it and the cuda FLIES ! no issues !