One perk of the factory Six...
One perk of the factory Six Pack setup is that you can bolt it on a stock-looking big-block that is anything but, and have plenty of fuel to feed whatever kind of beast you've built.
Just what is it about a Six Pack setup that gearheads find so cool? Yes, they are synonymous with some of Mopar's most powerful engines. Sure, they are one of the most desirable original options. And there's no mistaking their roar when you open them up. Still, few would argue that it's easier to tune a single four-barrel, and most would likely admit that you can get better performance that way.
It's not that they don't work well, because they really do. Even for a radical big-block, 1,350 cfm is plenty of air. It's just that on anything but a stocker (and even then sometimes), they can be a real hassle to get finely tuned. The major stumbling block with this setup is that it, well, has a tendency to stumble when you stab the pedal hard. Part of the problem is the outboard carbs use a fixed metering plate that doesn't have replaceable jets, and they are a vacuum design that lacks accelerator pumps. If the outboards come in too quick, you can experience a slight hesitation. If you hold them back a little, you can avoid the stumble but sacrifice power. Imagine trying to throw open a 1,000-cfm four-barrel with no accelerator pump, and you begin to understand the issue.
Now, if you add a performance camshaft, improve the heads' flow characteristics, and bolt on a free-flowing exhaust, you can count on having to fatten things up a bit in the fuel delivery department. Unfortunately, the only way to do that with the outboard carburetors' stock metering plates is to find ones with different size orifices (have fun with that one), or to carefully drill out the stock plates until you get to where you need to be. That will work, but it's a real pain in the rear to do, and you can't back up if you go too far with the drill size.
The factory nonadjustable...
The factory nonadjustable plate has a metering block gasket, then a thin, metal plate, another gasket, and finally the nonadjustable metering plate. By adding the Promax kit, you end up with a metering plate gasket and the adjustable plate.
Of course, you can't just ignore the idle mixture in this process. The correct idle settings are more critical than you think in order to get the outboard carbs on line smoothly. The center carb is easy since it has a normal metering block, and you can get access to the front idle screws easily, but to get at the rear idle mixture screws, you have to remove the carb. Again, you can do this, but you can imagine the frustration of trying to adjust a carburetor off the engine.
This was the situation the guys at Muscle Car Restorations encountered with Ed Nasser's T/A. Ed's 440 had been rebuilt with a set of Edelbrock aluminum heads and a Comp Cams 298/288 degree/.528 lift cam; tti headers were installed. While the carbs worked great during the Mustang dyno tune, they did stumble a bit out on the street, so Ed obtained a ProMax conversion kit made specifically for the '69-'71 Mopar Six Packs.
Promax manufactures billet replacement metering plates for the outboard carburetors that use standard Holley main jets, and a replacement baseplate that has angled idle screws that are accessible while the rear carb is mounted on the manifold. Ed also had the guys at MCR install one of the Promax billet metering blocks on the center carb.
As expected, when the car was hooked up to the O2 meter, it showed the carburetors were running a little lean and some adjustments needed to be made. The center carburetor was not that far off-just one size up, from a 65 jet size to a 66. But the outboard carburetors jumped from a staggered 78/82 configuration to 84s at all four locations. Now when you mash the pedal on this big-block, it shows no signs of timidity. It just goes!
The Promax metering plate...
The Promax metering plate uses standard Holley main jets; the rear carburetor's baseplate has angled idle mixture screws that allow the outboard carbs to be tuned as easily as the center one.
The factory presets the idle...
The factory presets the idle screws on the outboard carburetors and then puts a lead plug over them. Pull the plugs and you can adjust them normally. The screws on the front carb are accessible under the fuel bowl so you won't need a Promax throttle plate for the front carb, though one would make it easier to get at the front mixture screws.
The Promax baseplate is shipped...
The Promax baseplate is shipped without the throttle shaft or plates, so you will need to swap in the ones from your original base. Just don't forget to use a thread locker or stamp the ends of the screws so they can't come out.
With angled idle mixture screws,...
With angled idle mixture screws, there is no need to remove the rear carb to make idle adjustments; the rear carb can be adjusted while on the manifold with the engine running.
The billet metering plate...
The billet metering plate is a direct replacement for the nonadjustable (except with a drill bit) pot metal factory version. All you need is a supply of Holley main jets.
The factory metering plates...
The factory metering plates are not only nonadjustable, they also sometimes come with staggered jetting holes. On this plate, the driver's side was a number 82 size orifice, but the passenger side was a 78. As it turns out, Ed's 440 preferred the same jetting on both sides.
Promax also machines a billet...
Promax also machines a billet metering block for the center carburetor that will allow you to build your own performance fuel curve. This one worked great right out of the box on Ed's 440. the Promax version does not have a ported vacuum nipple for vacuum advance. If you want to connect the vacuum can on your distributor, you will need to stick with the stock metering block.
Though it's not necessary,...
Though it's not necessary, if you grind a little notch into the driver's side of the fuel bowl, it will be a little easier to access that idle screw.
Just don't grind too deep...
Just don't grind too deep.