The OEM 4300 series carburetors were equipped with a single fuel bowl of insufficient capacity, known to cause an engine to stumble when accelerating out of a corner. They were also equipped with crudely designed booster venturis having big tubes that dumped the fuel into the air stream. The air horn casting had a reputation for warping all too easily during assembly of the carburetor.
The Autolite 4300A carburetors installed on the 1971 M code engines were rated 630 cfm by Ford, but if measured using the same parameters used by Holley their flow would be rated approximately 480 cfm. They were small carburetors which I like to describe as “clogging” the engine. They were also emissions tuned. The 1971 M code engines were down by 25 horsepower compared to what they could have made if a performance-tuned Holley rated about 600 cfm had been installed.
The Motorcraft 4300D carburetors installed on the 1972 through 1974 Q code engines were rated 750 cfm by Ford, but if measured using the same parameters used by Holley their flow would be rated approximately 600 cfm. The 4300D carburetor didn’t “clog” the engine as the 4300A version did. But 1972 was the year that emissions tuning took the most pronounced step in severity. Compared to what they could have made if a well-tuned Holley rated about 600 cfm had been installed the 1972 Q code engines were down 75 horsepower, the 1973 Q code engines were down 90 horsepower, and the 1974 Q code engines were down 80 horsepower.
Understanding the performance sacrificed in using the Autolite/Motorcraft carburetors De Tomaso installed 600 cfm Holley carburetors on the engines installed in the early Push Button Panteras. They also installed 650 cfm Holley “double pump” carburetors on the engines installed in GTS, Gp3, GT5 and GT5-S Panteras.
The 4300A version is too small, they aren't worth modifying. The 4300D would be a better candidate for modifying because it flows more air, somebody skilled with these carburetors could revise the 4300D air/fuel ratio calibration, and recoup most of the horsepower these carburetors rob from an engine. I had a "carburetor" guy who did this for me back in the 1970s & 1980s. However finding a person to tune a Motorcraft 4300D today is difficult. And even after recalibrating the carburetor it shall still have the fuel stumble issue caused by the small fuel bowl; so paying to have a 4300D modified seems like a bad investment to me.
Thus for anything outside of a 100% factory restoration I would replace an OEM carburetor with a performance tuned carburetor of 600 to 680 cfm air flow capacity (flow rated using Holley criteria). It should be tuned for a richer A/F ratio (street performance), and it should feature vacuum secondaries, high capacity dual fuel bowls, an electric choke and most importantly annular booster venturis. Annular booster venturis provide a stronger “signal” for more precise fuel metering (especially at low rpm), they atomize fuel better (smaller droplets flash into vapor more readily) and they mix and distribute the fuel more evenly in the air stream – thus they promote a more consistent air/fuel ratio from cylinder to cylinder. Annular booster venturis close the gap in performance between carburetors and throttle body fuel injection.
The carburetors I would choose from include:
- Summit Racing p.n. M08600VS
- Demon Carburetors p.n. 1282020VE (discontinued but can still be found)
- QFT SS-680-VS custom ordered with annular booster venturis ($125 extra)
Since your Pantera's engine was originally equipped with a “spread bore” carburetor it shall require a replacement “square bore” intake manifold for the replacement carburetor. If you choose to utilize an OEM square bore cast iron intake manifold you will need to increase the diameter of the four carburetor butterfly holes to 111/16 inches diameter (1.69 or 1.70 inches). Another option is the Edelbrock #2665.