AVS Mixture Screws Plugged
- Thread starter adelmo
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Well just heard back from Mopar Guru friend and sounds like there were several versions including mine with plugs.
Curious what other 68-69 440 owners have with this carb.
Curious what other 68-69 440 owners have with this carb.
My original 1968 (built 11/67) RR 383 has the same carb body. No mixture screws - unlike the new AVS 2 I've been playing with.
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The 1969 FSM shows the plugs on the AVS too. The left hand thread screw in the middle is used for idle adjustment.
There are screws under there. You just have to go get them.
I had my 4429 restored and the first thing I did after getting it back was ordered aftermarket brass mixture screws like the 69-up versions typically had. Much easier to get it dialed in at idle.
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The mixture screws were plugged, at final assembly after flow testing, to achieve the contracted mixture requirements of Chrysler Corp. The "idle mixture screw" was located above, as shown in your pix, and was a left hand thread configuration, which was an adjustable air bleed, when turned in, increased the mixture richness, by decreasing the air flow thru the idle circuit. I believe this feature was for 1968 model year. The following year, the system was revised to eliminate this feature, but added added an "adjustable air bleed" circuit to fix an off idle condition for emission control requirements. This "adjustable off idle air bleed" circuit was set at the factory and sealed...it was NOT an adjustment to be changed by the owner, as it was set to a differential air pressure value between the idle air flow and off idle air flow pressure.
The plugged passages shown in your pix, could be carefully removed, to expose the mixture screws inside the body casting, accessible via a small thin blade screw driver. USUALLY, turning the hidden screws COUNTERCLOCKWISE approximately 1/4 turn, resulted in a smooth idle and eliminating a stumble condition.
It is important to realize that ALL of the idle mixture fuel is derived thru the primary booster venturii clusters via fixed, non-adjustable drilled orifices in the booster venturii via drilled passages in the main body. The primary metering jets and step up rods control fuel flow only when the boosters are feeding fuel and DO NOT conttol the idle fuel flow. This is true for both the AFB and AVS carbs.
Hope this helps your understanding of the carbs operation.
BOB RENTON
383man
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I agree completely as I was going to say the smaller screw in the middle is into the idle air bleed that feeds into both side idle circuits. They use left hand threads so most who dont know what they are and turn them like they would and idle mixture screw get the same results. That is turning it clockwise (in) most think it will make the mixture leaner as a normal mixture screw will. But with left hand threads turning it clockwise turns that screw out which lets more air into the air bleed circuit and leans out the idle. And vice versa as turning the screw to the left counterclockwise will turn that screw in and richen the mixture since its actually turning that screw in cutting the air bleed off some. There was also some Carter four barrel's that had a bigger screw in between the mixture screws and that screw was an idle air screw as it set the idle speed by letting more air into the eng since they had no idle speed screw on the linkage to open the throttle blades. But that screw is bigger as its twice the size of the mixture screws or even bigger. I have a good pic of the cluster on them showing the idle restriction but I cant post it since I did not renew my hosting place for my pics. I talk on face book alot and you dont need a hosting place to post pics on there. Sorry about that. Ron
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With regard to "a bigger screw between the normal mixture screws", yes this was the idle speed conttol, as it functioned as an "idle air bypass" to allow the throttle plates to be almost closed. This provided a reduction of fuel delivered thru the off idle transfer circuit (idle transfer slots) and to reduce the possibility of the primary boosters feeding fuel due to, possibility, a high lift/duration cam, needing more idle air for smooth operation. Chrysler used the idle air bypass design on the Carter 3854S carb used on the 1965 HP 273/235 Formula S Barracuda as well as some GM Pontiac GTO applications. Just continuing the information presented.....
BOB RENTON
Dragon Slayer
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Throttle valves are not almost closed on air bypass they are closed. 66-67 Street Hemi also used this design as did Cadillac.
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On the Carter 3854S I used on my old '67 Belevidere II, with the LA 318 engine, had exrernal spring loaded throttle stop screw on the linkage side, that prevented the throttle plates from going completely closed (to prevent sticking). The hot idle speed was adjusted via the large idle air bypass adjustment. I had purchased the Carter 3854S carb, new, from the Pittsburgh Pa Carter distributor and the Mopar single plane 4 bbl manifold ftom the local Plymouth dealer. The carb came with the throttle stop screw installed and adjusted with the note to use the idle air bypass adjustment to adjust the idle speed. After a minor mixture adjustment with the metering rods, the carb performed quite well. I've no experience with either Hemi or Cadillac engines noted, to speak to that end.
BOB RENTON
Dragon Slayer
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You can add them to most carbs if the cast lug is there and the lever has the stop attached. According to the Carter Form 5484 the 3854 an 53 for the 273 motor only had the Air screw and did not have the idle screw or spring in the parts list.
I have the equivalent NOS Marine carb for the 273 motor (posted in for sale section). It has the cast lug not fully drilled and not tapped and no screw. The lever does have stop attach lever on it though. So it would be a simple mod that could be done.
My service book does not have modification for it, but I do have a number of CAD/PON and Hemi carbs an none of them had idle mixture screws. Frankly, the pri valve are cut for a 15 deg angle and while a good unworn carb they are sticky when shut solid, they don't get stuck. Plus they are directly mechanically opened by the throttle cable/lever.
Hemi carbs don't have that stop lever, but the Pon do. The CAD don't have the cast lug or lever.
I have the equivalent NOS Marine carb for the 273 motor (posted in for sale section). It has the cast lug not fully drilled and not tapped and no screw. The lever does have stop attach lever on it though. So it would be a simple mod that could be done.
My service book does not have modification for it, but I do have a number of CAD/PON and Hemi carbs an none of them had idle mixture screws. Frankly, the pri valve are cut for a 15 deg angle and while a good unworn carb they are sticky when shut solid, they don't get stuck. Plus they are directly mechanically opened by the throttle cable/lever.
Hemi carbs don't have that stop lever, but the Pon do. The CAD don't have the cast lug or lever.
383man
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Myself I have seen many pics of the Carter carbs that used the large idle air screw and learned all about them back in high school in the 70's. But I have never had one on any of my cars. Since I was an auto tech for a living I always tried to learn of all the different carb systems in case I have to work on one. It does kinda make sense that the carbs with the large idle air screw to set idle speed would have some sort of stop to keep the throttle blades from closing completely and maybe sticking. Keep them open just a tad but as I said I really never actually worked on one to see how far the throttle blades shut. Its a good idea so the idle transfer slots can be set just right and you can set idle speed without messing with the throttle blade settings. Ron
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