Carb Mods - 1970 Plymouth Big Block Duster Project

I've always favored Holley carbs but this time around I decided to try Edelbrock/Carter AFB's for a few reasons: While they are arguably considered (by some) to be less performance oriented than a Holley, they're also less expensive. If you understand how they work, they're simple to tune with less chance for mess and leaks. If they were good enough to power the Hemi and dual quad 409 back in the day, well...you decide. Remember, this is a street motor, not a racer.

Also, it's recommended that Holley carbs be boost referenced so that the power valve will work properly and not cause the engine to go lean. A blown engine going lean under hard throttle is bad, bad, bad. Boost referencing any carb requires special mods.

I think the Carter's have a slightly more retro look on top of a blower in an inline configuration too. So, with all this in mind I chose a pair of 750's (Edelbrock #1407) and purchased them, rebuilt, through National Carburetor (via Ebay where I saved even more over buying from their website). Edelbrock actually recommends a pair of 600 cfm carbs on a blown 440.

They appear 'like new' with all new fasteners and hardware. A pair of these cost less than a single, new, blower calibrated Holley. Your logic may vary.

Buying these doesn't mean you shouldn't modify them right out of the box. A richer baseline tune is a good idea to prevent the aforementioned potential meltdown on an overly lean fuel mix. The recommendation I found is to go 1 stage richer on both the primary and secondary jetting as a starting point.

The 1407 carb comes stock with .113" mains, .107" secondaries, .071" x .047" metering rods, and orange (5" hg) step-up springs. The step-up springs control at what vacuum level the metering rods move to provide fuel enrichment under a load - similar to a power valve in a Holley.

One step richer calls for .116" mains, .110" secondaries, and .073" x .047" metering rods. I installed the stiffest, plain/silver (8" hg) step-up spring. This would provide fuel enrichment under load sooner than the stock spring.

Also upgraded to larger .110" needle/seats.

Additional tuning information can be found in this Edelbrock Manual.

While I installed different jetting, I also stripped off all the unnecessary parts - the choke system - which is not needed.

All the manual choke brackets, fast idle cam, rods, plates, and levers were removed. I later also changed out the inlet fitting for one to accept -AN hoses.