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Decoding Q-jets

 
As for a quick decode, conside Q-jet model number 17056259. Q-jets all begin with either 70 or 170, indicating a Rochester product (the 170 carbs are the newer ones, usually post 1974, though trucks used the 70 carbs until the 1980's; big difference is that the primary rods don't interchange between these two series carbs). The next two digits indicate the year. Actually, the 6 is the last digit of the year, and the 5 is just different from the 2 that you would find in that place on a 1966 Quadrajet (I believe the 1986 would probably have a 7 in that place) That leaves the last three digits, which usually distinguish the particular application of the carb for that year. The second to last digit usually indicates the manufacturer, with 5 being Olds (but just like that i before e, except after c thing, there are exceptions, especially in the mid-'60's). For example, most 350 4bbl Olds carbs ended with 250, and the Toronado engines ended with 252 for a long time. The standard big-blocks had the 251 carb, and the 442 and W-31/W-30 carbs would end in 254 to 259. No, this example is not a 1976 W-30 carb, though. After 1974, to meet the more stringent emissions requirements, makers specified specific carbs for engines in particular models. So while every 1971 350 4bbl carried the carb # 7041250, in 1976 there were different numbers for the Cutlass carb, the Delta carb and the Omega carb. As to whether it's a good choice for a performance buildup, it's not a *bad* choice, though I know I have built up a better selection of the 70 series primary rods than of the 170 series rods. Whatever you use it for, you'll likely have to rejet it and use different rods; with the 170 series carbs, GM seemed to favor larger jets, but thicker primary rods, which netted less fuel flow. Jets and secondary rods will interchange between series. A few Carter Spreadbores ended up on some Cutlasses in the early 70's. Rumor has it that Rochester could not produce the amount of Q-jets needed so they subcontracted Carter to pick up the slack . You can id one of these pretty easily. On the side of the carb it will say Quadrajet by Carter instead of the usual Rochester Quadrajet. [ Thanks to Jeff Easton, Bob Barry, Chris Smetana, Steve for this information ]
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