How to speak Rochester


Often people have wondered how I can pick up a Rochester and just rattle off the vital specs or, when asked which is the correct Q'Jet for a 72 YS Goat can spout 7042264. Keep reading and so can you.

This discussion is limited to the period from 1962 to 1975 for several reasons: in 1963 Rochester part numbers statred to make sense and by 1975 GM was producing many different engines (some eminently forgettable), the feds and California each had their own, and the numbering system changed in 1976 so it is a good place to stop.

To begin with, Rochester separates carburetors into groups - a 2GC goes in the center of a tripower, a 4MC is a Q'Jet. Right.

The groups are easy to decipher: the first number indicates the number of barrels (one barrels leave the number off), the next letter is the venturi type: In this period Rochester produced B (one barrel), G (two and four barrels), and M (Monojet/Quadrajet) series in this period. The last letter indicates if a choke is used: no letter for none or manual choke (tripower end carbs were 2Gs), C for an automatic choke with the coil mounted on the carburetor and hot air tubes leading to the carb., V has a vacuum brake and the choke coil is mounted on the manifold with a linkage to the carburetor. After the oil embargo and Iran things get messy (MME, SE, etc.) so we will stop before that happened.

Thus a 4GC is a four barrel, G type carburetor with automatic choke having the choke coil on the carburetor.

Of course these are not the numbers found on the tag or float bowl but are handy in finding the right part of the service manual and following what comes next.

Up through 1975 all factory Rochester carburetors were identified by a seven digit part number either stamped on a tag under one of the float bowl screws (1955-1965) or stamped into the side of the float bowl (1966-). The first two digits are 70 indicating Rochester Products.

By 1963 someone at GM got the bright idea of coding the nexttwo digits to the model year so 1963 became 23 and 1964 became 24. Thus a 1966 Carburetor p/n starts with 7026xxx.

Now this must have made too much sense so GM quickly found a way to foul it up: In 1966 California started requiring the infamous Air Injection Reactor or "air pump". These needed a different carb and to differentiate these from the normal sequence (7026xxx), these were labeled 7036xxx. For some unknown reason, '67 Ram Air GTOs and Firbirds also used 3: 7037271 (GTO) and 7037276 (Firebird). Thus when 1970 came along, instead of reusing 2x or 3x, Rochester jumped to 7040xxx which held until the 1976 digit increase (1975 was 7045xxx, 1976 was 17056xxx).

With the 1959 model year, the last three digits began to mean more than just a consecutive series. Numbering still began with 000 but decades were awarded to each engine manufacturer:

Chevrolet 	x0x x1x x2x 
Cadillac 	x3x
Buick 		x4x
Oldsmobile 	x5x
Pontiac 	x6x x7x
Non-GM 		x8x
Special 	x9x

Thus a base 1963 389 automatic 2GC was p/n 7023060 and a 4GC for a 1966 Buick 401 was p/n 7026040. Note: this is true only for Rochester - Carter AFBs used a different numbering sequence.

In 1966 what had been a trend became a code as manual transmissioned cars were usually given odd numbers and automatic units became even numbered. Generally the most common engine/trans combinations were given the first numbers while odd-balls started higher. End carburetors in tripowers generally got x78 and x79 (Pontiac), 055, 056, & 057 (66 Oldsmobile), .

For many years, all numbering started with 0xx, but as carburetors proliferated 1xx started to be used for the overflow while keeping the same decade arrangement. For example 1964 421 tripower end carb is 7024079 while the slightly different 389 tripower end carb (smaller jet) is a 7024179. Type B or M one barrels, also were usually 1xx units though some (e.g. 1968 Pontiac 7028065 monojet, 1971 Vega 7041023) used the 0xx series.

The next change came with the introduction of the M type Quadrajet carburetor to Chevrolet in 1965, Pontiac on the OHC-6 and Oldsmobile in 1966, and across the board in 1967. Since a large proportion of Rochester's output was these, the QJ was assigned 2xx with otherwise the same assignment. Thus by 1967 another order had emerged not originally intended: 0xx were 2Gs, 1xx were mostly monojets, and 2xx were Quadrajets.

To confuse matters, with the 1969 model year, the 703xxxx designation for emissions carbs was dropped and California carbs began to use the following series when a special carburetor was fitted:

5th digit (series) 	0 became 3 or 9
 			1 became 4 or 8
 			2 became 5 or 9 

Thus a California QJ for 1970 400 B car automatic was a 7040562 while the Federal QJ was a 7040262. This numbering continued through 1975, however in 1970 the famous "corporate engine" mixing started and the number would carry the p/n of the parent factory. Thus if a 1972 Pontiac Ventura II used a Chevrolet 307 engine, the p/n would be 7042100 indicating Chevrolet.

Now you have the simple guide to breaking the code such as a 7041262:

 	70 - Rochester Products 
 	41 - 1971
 	 2 - Federal Quadrajet
 	 6 - Pontiac
 	 2 - base automatic.

Since there were no 350 four-barrels in 71, you can say "1971 49-state Pontiac 400 Automatic" and be 98% sure of being right while watching the jaws drop. Easy.

One word of caution, Delco produces many "service replacement" carburetors each year. If you walk into a dealer and ask for a Quadrajet for a 1971 Toronado 455 cid, what his book will show is a 17054934. The "1" prefix is a givaway that something is not right (used after 1975). Be warned.

Final note: for more information on the care and feeding of Rochesters than you ever wanted to know, I recommend Doug Roe's excellent Rochester Carburetors published by HP Books in Tucson, Arizona.