This is a brief survey of the Rochester Quadrajet carburetor.

Quadrajets were used as original equipment on many vehicles from 1965 to 1990. Over 100 million have been produced.



1966 Rochester Quadrajet used on 327 Chevrolet engine with manual transmission, no. 7026203 - number is on the blue 'bottle cap' visible on the throttle side

Design

The QJet is a large 4 barrel carburetor that has a small primary side for fuel economy and good emissions and a large secondary side for good performance. QJets have a maximum airflow of from 750-800 cfm. Because the secondary side of the Qjet opens according to the airflow requirements of the engine, the same basic carburetor size can be used on a large range of engine sizes. The basic 750 cfm QJet casting is used from 231 CID to over 400 CID - the big secondary air valves open only as the engine breathes. For example, on a mild 350 Chevy the air valve will never open all the way - because the motor can't use more than 600 cfm or so.



1980 Chevrolet truck Quadrajet, M4MC type (hot air choke), no. 17080213. M4MC Quadrajets have an integral choke and a different airhorn design; also, the throttle shafts are longer than in 4MV carbs. Starting in 1968 carb numbers were stamped on the driver's side, near the back.

Identification

There are three basic types of QJet: the original 4M (starting in 1965), M4M (1975 onward) and E4M (from 1979). All QJets had automatic chokes, of which there are two basic types - integral (choke coil on the carb) and remote (choke coil on the manifold). 4MV carbs have the choke coil on the manifold and 4MC carbs have it on the side of the carb in a housing. An M4MC carb has a choke coil on the carb; and M4ME carb has the same type fo choke coil, but with an electric heater element in the choke coil.

Choke parts, primary metering rods and throttle shafts are not interchangeable between 4M and M4M carbs.



Oldsmobile M4MC Quadrajet carburetor, showing the front fuel inlet used on BOP (Buick-Olds-Pontiac) carbs

Evolution

1965-66 4MV, 4MC: hydraulic piston damped air valve; originally came with diaphragm-type fuel inlet valve - now upgraded with needle/seat type; 4MV (remote choke) used on Chevy starting with 1965 396; Oldsmobiles used the 4MC on Toronado in '65

1967-1971 4MV, 4MC: vacuum break used to damp air valve instead of piston; fuel inlet thread is 7/8"-20 on most carbs

1972-1979 4MV carbs are used on cars until 1974; trucks until 1979; fuel inlet thread changed to 1"-20 in '72

1975 M4MC carbs have different metering rods from 4MV and have a carb-mounted choke; used on passenger cars and some trucks from 75-up

1975 M4ME electric choke carb used on Cadillac

1980 E4ME computer controlled carb

1985 M4MED electric choke carb with dual capacity pump solenoid used on Federal trucks and various Canadian vehicles

1985 M4MEF with adjustable secondary air bleeds used on 454's with HD emissions, E4MED and E4MEF in California

17088152 the last OEM Quadrajet number (an E4ME)

A complete manual for Rochester carburetors is available at www.newagemetal.com, also the Old Car Manual Project has many carburetor manuals.