Rochester Quadrajet Carburetor - Quadrajet Quotient - Junkyard-Crawler Tech

Pontiac used a unique Quadrajet Model 4M for specific applications during the '71 model year. Retaining the 13/32-inch diameter primary, this unit lacked the outer velocity boosters found in all other models. Though primary airflow increased towards 230 cfm, the reduced nozzle signal from the missing boosters had a negative impact on throttle response and emissions.

The primary diameter of the Super Duty 455 Model 4M and all Pontiac Model M4Ms from '75 forward was increased to 17/32 inches, and airflow jumped to approximately 210 cfm. There is a noticeable bulge that protrudes outward from the bore wall. Seen at the 8 o'clock position here, this characteristic can be used to quickly determine primary airflow capacity at swap meets or salvage yards.

This tab located on the secondary air-valve shaft is used to restrict airflow in applications where maximum capacity isn't needed. A slight amount of material can be removed to increase airflow capacity for high-performance engines that require additional airflow. In these instances, the rear portion of the secondary air valves should open 80-90 degrees from its resting position, which should yield a maximum secondary-airflow capacity of roughly 600 cfm.

Though 20.4 inches is the accepted pressure standard for rating four-barrel carburetor airflow, some aftermarket carburetor manufacturers rate their carburetors at 28 inches. So it is difficult to directly compare flow values without knowing the stated pressure. This HP-series Holley measured 950 cfm at 28 inches and 820 cfm at 20.4 inches.

Other than a few early Quadrajets that utilized a stamped-steel disc, the application casting number can be found on the driver's side of the main body.

Pontiac engines used a divorced-style choke with a thermostatic coil mounted on the intake manifold exhaust crossover through '72.

From '73 forward, the thermostatic coil was mounted onto the carburetor main body and functioned from preheated air.