Been messing with several different AFB from different manufacture and comparing to the Hemi carbs. One thing that struck me is how different many AFB carbs even across manufacturer. Buick/Cad/Chevy/Pontiac all had different approaches on Throttle bore sizes, main and primary venture size and design, etc... Even comparing GM Big Block they all seemed to take a different approach using the same basic AFB design. I understand carb basics, and even some of the theory, but have not been able to find any info on why so many subtle difference and what were the engineers after?

First, the hemi stayed relatively the same especially manifold, heads and exhaust. So why all the changes across year with jetting. They started with big stagger on Front carb secondary and ended up with smaller stagger on rear carb secondary? They had to use deflector tabs which ultimately should have an adverse effect on CFM. Was the manifold that bad as far as even fuel distribution to all the cylinders?

The other interesting item is the shorter primary main venture that they seemed to use. It was sunk into the carb body for the hemi. They then used a larger body primary venture with a long booster body. Most of the other manufactures did not do this for their Big block and or dual quad motors. The main venture was fuller and that resulted in a shorter primary venture. What is the difference in carb performance?

I understand Venturi diameter and how it effects flow and pressure, but could not find any info on venturi length and it's effect. Picture shows a hemi front against a Pontiac 4 barrel AFB. You can see the difference in primary side main venturi and also primary (booster) venturi. Both are rated the same for Throttle area and Venturi area. Even the internal machining was different on the Pontiac. Same diameter but after the venturi they opened the booster more.

Anyone heavy into racing these Carter's in the stock class back in the day? What approach for a basic stock Hemi motor did you take? Thanks

I race a pair of AFB's in nostalgia super stock. I use 750's the ones built by Federal Mogul. The extended booster I have read is to give the booster more signal at high RPM. I actually was planning to try and put a sleeve on my boosters to extend them, to see if it helps this winter. My engine has twin O2's and the engine is always trying to lean out at higher rpm's. AFB's have a small float bowl, but I have converted mine to dual feed lines and cut windows in the seats to improve fuel flow. I wish the AFB's had screw in air bleeds, which is also on my list for the experiment on the carbs this winter. Would like to hear from anybody who has done mods like this.

air bleeds are possible - use the emulsion jets - here is a pic I had with a carb that was done this way

can not make it out from the pictures, any idea what orifice size they were?

You start with the stock AFB bleed size and work up or down from there - you end making many of your own bleeds sizes as you chase the fuel curve - its not easy to do and the gains are small.

The Primary booster (small one) also called Auxiliary booster by some has to have its outlet at the smallest portion of the Main Venturi which is built into the body. By doing this your using the lowest pressure point of the main venturi to trigger the primary booster and get the greatest gain. The real difference is the hemi carb has the main venturi lower (closer to the throttle blade). Therefor the primary venturi needs to be longer to place the outlet inline with the main venturi choke point. This basically creates a larger air plenum area above the bore. Most other AFB have a longer distance between venturi and blade. Since it sits higher, the primary venturi is shorter to again keep the outlet at the choke point with lowest pressure. Fuel well machined surface is at same height between bodies.

Looking on line last night I stumble on the chevy documents and they used the same configuration as the hemi for the Dual Quad 409 and the single 4 for the high HP 327 Corvette.

I am just not sure if in theory that mod helps lowend torque or high end RPM power.

As far as your leaning at high RPM, isn't that by design to get more power? At WOT as the fuel bowl level lowers you expose more air holes on the secondary pick up tube which leans the mixture by design for high RPM power. If too lean, maybe a bigger secondary jet would help.