The Rochester Quadra-Jet was a very popular carburetor on many GM vehicles from the mid 60s to the late 80s. The first was in 1965, on large Chevy V8s. By 68, the Q-jet was GM's main 4 barrel carb. The OEM numbers are located on the driver's side of the carb, near the throttle linkage. The numbers run vertically.

The Q-jet oem numbers go like this:

70YYEDT or 170YYEDT

70: Rochester carbs started with either 70 (before 1976) or 170 (after 1976).

YY: The year code. The years are a little convoluted, but probably the easiest part to remember.

The code for 1965 would be either 25 or 35, 66 would be 26 or 36, 67 would be either 27 or 37, etc. 1970 is 40, 72 is 42, 75 is 45. You get the idea. The 80s are easier because they're usually straightforward. 81 is 1981, 84 is 1984.

E: The emissions code. 2 is for 49-state emissions, 5 is for California & high altitude.

D: Division. 0, 1 and 2 are Chevy, 3 is Cadillac, 4 is Buick, 5 is Oldsmobile, 6 and 7 are Pontiac. Rochester doesn't always follow this rule.

T: Transmission. Even numbers are automatic trans, odd numbers are manual trans. Rochester also doesn't always follow this rule.

This guide is just that: a guide. It is not set in stone, because it is a documented fact that Rochester didn't follow these rules 100%. It is for Quadra-jet carburetors only. Rochester also had different numbering systems for their other carbs besides the Quadra-jet. Our thousands of pages of information on this comes straight from Rochester.

Keep in mind that just because the OEM numbers between two carbs may be different, it doesn't mean they are different carbs. It's very possible to have two carbs, their date is 2 years apart, one is for Buick and one is for Chevy Truck, and one is for automatic, the other for stick, and the carbs are identical in every single way, even down to the jets and metering rods. It's much more common than you might think. Also, many people think they have to have a carburetor with a certain exact OEM number. This can sometimes be very very hard. Most people don't have to have this, they really only need a carburetor that crosses over the same and is identical. You realistically only need an exact OEM number if you are restoring a car back to original condition to enter into a car show.