AVS carb questions

Fuel and Air Systems

  1. FirstHemi

    FirstHemi Member

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    Can someone tell me what the difference is between the AVS carburetor for an automatic transmission and a manual transmission?
    Also can someone tell me what the difference is between the carburetor for a 383 and the 440?
    From what I see in my service manual AVS 4736 is for 383 manual transmission
    Avs 4732 is for automatic transmission, and
    Avs 4737 is for 440 manual transmission
    Avs 4738 is for 440 automatic transmission
    Trying to understand what makes them different
    Thanks
     
  2. RemCharger

    RemCharger Well-Known Member

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    Sizes of everything would be the first thing I would think of.
    The bare castings might be identical? I would think.
     
  3. Tony Tee

    Tony Tee Well-Known Member

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    My 4 speed AVS has a dashpot. To slow down the rpm drop for shifting easier. I believe?
     
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    • GTX JOHN

      GTX JOHN Well-Known Member

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      383 1 9/16 Primary (625)
      440 1 11/16 Primary (750) generally
      Dash Pot affects opening of secondaries as well.
       
      Last edited: Jan 22, 2022
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      • maxwedgechar

        maxwedgechar Well-Known Member

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        Automatic would have provision for the kick-down rod.
         
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        • Dave6T4

          Dave6T4 Well-Known Member

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          Sometimes, different calibrations (jets or metering rods).
           
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          • HALIFAXHOPS

            HALIFAXHOPS MSGT(ret) and MOPAR only distributor rebuilder! FBBO Gold Member

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          • 66Satellite47

            66Satellite47 FBBO Gold Member FBBO Gold Member

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            Like above have said the sizes of venturis and throttle blades varied with the engine size. Also, IIRC the metering rods/jets and possibly the air valve spring were different.
             
          • AR67GTX

            AR67GTX Well-Known Member

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            For a given engine, the differences would probably be very subtle, slight change in Venturi booster, maybe air bleed size, primary rod diameters. Even a single, seemingly minor change would require a different part carb number.
             
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            • HALIFAXHOPS

              HALIFAXHOPS MSGT(ret) and MOPAR only distributor rebuilder! FBBO Gold Member

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              Probably similar to what the difference in distributor curves are Manual usually rev up faster.
               
            • RJRENTON

              RJRENTON FBBO Gold Member FBBO Gold Member

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              IMO.....the basic castings are similar but application vs application or auto trans VS standard transmission, the differences between the two are sometimes very subtle. Assuming for the same engine, the standard trans application will usually include an externally mounted DASHPOT, that slows the closing rate of the primary throttle plates to prevent a large RPM drop when shifting (other than WOT) and a subsequent lurch upon clutch re-engagement. USUALLY, beside the obvious differences in primary and secondary metering jets, the primary metering rods and step up piston springs will likely be different, but the most overlooked items are the primary booster venturii assemblies, which control ALL of the idle and off idle fuel transition circuits and are not adjustable. 1968 AVS designs had a single idle mixture adjustment screw a NON ADJUSTABLE off idle air bleed. Later models went back to dual idle mixture screws, but maintained the non adjustable off idle air bleed. Other subtle variations include the size and location of the off idle fuel transition slots and their size. As emission regulations became more stringent in later years, accelerator pump well size and pump stroke changed to accomodate emissions and transmission. Sometimes, throttle plate bevel angle changed to accomodate the engine's air flow fuel flow transition from idle to and including main fuel flow.....depending on transmission used. To be specific, careful disassembly and measuring all the components of the carb is the only true way to discern the differences between applications. Others will say it doesn't matter....but it depends on the individual's wants or needs.
              BOB RENTON
               
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              • HALIFAXHOPS

                HALIFAXHOPS MSGT(ret) and MOPAR only distributor rebuilder! FBBO Gold Member

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                Thanks again Bob. Knoledge on these should be passed down.
                 
              • GTX JOHN

                GTX JOHN Well-Known Member

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                If you look at the booster venturis you will see some tabs on some of the
                to correct Air Flow. I look for ones without those if I use aftermarket manifolds.
                 
              • Dragon Slayer

                Dragon Slayer Well-Known Member

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                I have charted this for AFB and just started doing it for AVS. As stated the 440 HP had the larger 1 11/16" Primary bore while the 340/383 were 1 7/16". The calibrations between 340 and 383 are very close. And from what I can see the Man seem to be leaner than Auto which seems like a reversal from AFBs.

                If you want to understand the detailed changes the carter service manual has the calibration data on the air bleeds, jets, etc... up through 69. After that MOPA probably did not pay to have that data and the service document are only about setting, not the detailed calibrations.

                AVS only have primary venturis the secondary with the rapid opening of the air valve above the bore doesn't. You have a calibrated discharge tube with various sized hole. The other issue is the air bleed for the secondary is in the top, so having the correct top effects secondary calibration.

                The dash pot does not hold open the secondary. It is an emission item. It prevents the primary throttles from fully closing on a throttle closed slowing down situation with transmission in gear so high RPM motor. IF the throttle blades went full closed in this circumstance the motor would go lean, misfire and have unburned fuel exit the exhaust. So it keeps primary valve open enough to prevent this.
                 
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