Quadrajet Problem Solving > Quadrajet Parts and Numbers
Restamp 7040256 ID
4speed455:
Cliff Ruggles:
The Olds Q-jets thru 1974 will have small MAB's and smaller jets in them. They used APT to fine tune part throttle A/F on the primary side, mostly for emissions then capped it off to prevent tampering.
Before you can do any carb tuning you'll have to make all the decisions on the engine build. I've built thousands of Olds carburetors, matter of fact just finished up a W-30 carb a few days ago. I've found that most of the Olds factory "high performance" early carburetors used very small MAB's and in 1970 with the manual trans carbs they are really tiny as they eliminated the power piston and primary metering rods then ran them straight off the jets. I suppose the factory didn't think there would be enough vacuum with the really long duration camshaft. They were wrong for the most part and over the years I've built a good many custom early Olds Q-jets for the 1970 455 manual transmission W-30 engines and they worked flawlessly.
Those carburetors are usually missing and difficult if not near impossible to find anyhow, plus pretty expensive when you do find one........Cliff
4speed455:
Will this baseplate with the larger holes and idle bypass work well for a mild cam stock 455 or will I have problems getting it to idle? I have seen similar baseplates that are milled the same on the underside and drilled inside primary throttle bores. They were both on a 7029202 Carter carbs. So it looks like it is a Carter made Chevy base plate.
4speed455:
So I got to the part of the book where they talk about adding idle bypass and drilling holes in the main body if they are not present. My baseplate has all the holes drilled in it for idle bypass but my main body does not have holes. So it looks like I do not have idle bypass on this carb, just the holes in the primary throttle bores next to the transition slots.
Cliff Ruggles:
Idle bypass air is easy to add you just have to drill holes in the voids and make sure the airhorn has slots to allow incoming air to get to those voids. It's not fussy to drill the casting and "notch" the airhorn if needed.
Might want to try the carb without bypass air and see if it actually needs it. For a "stock" cammed engine it may be fine without it......Cliff
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