Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
What does idle mixture screws controls?
BOP455QJ:
I'm running a 1974 Oldsmobile 455 Quadrajet on my 1970 buick 455 but I'm using a more modern base (a 79 I think) because the original one is screwed. The car runs perfect,the only thing is that the idle goes completely smooth with each mixture screw opened 8.5 turns what bother me a bit because seems to be a lot. With the idle screws opened all that I have 20" of vacuum and my exhaust doesn't smell to gas, If I start to close the mixture screws the exhaust will start burning my eyes which makes me think that maybe is causing a lean misfire, and the vacuum drops to 19".
So, what does the mixture screws controls on our quadrajets? Fuel quantity, Air Quantity or both ?
Cliff Ruggles:
A later baseplate would have to be from a Marine carb or Service Replacement to fit a 1974 Olds, those are early style carbs. I may have fine pitch metric thread mixtures screws which are roughly two to 1 and it may also have tiny holes under the screws, so having to back them out that far is not unusual.....Cliff
BOP455QJ:
--- Quote from: Cliff Ruggles on June 06, 2019, 04:03:56 AM ---A later baseplate would have to be from a Marine carb or Service Replacement to fit a 1974 Olds, those are early style carbs. I may have fine pitch metric thread mixtures screws which are roughly two to 1 and it may also have tiny holes under the screws, so having to back them out that far is not unusual.....Cliff
--- End quote ---
Actually If I'm right this was from a 1979 chevy 350 Quadrajet, My 74 quadrajet is the one with the Aneroid APT and looks like any base from a later quadrajet fits it ;D
Cliff Ruggles:
A 1974 Olds is not the aneroid APT model, they were made in 1975 and early 1976.
If that's the case a 1979 Chevy baseplate would physically fit on it, but missing one attachment screw hole (they used 3 in the 1979 and later units).
I don't recommend switching baseplates around on those carburetors. Although they may bolt up there are more often than not differences in them that you may not notice that could make for a less than desirable end result........Cliff
BOP455QJ:
--- Quote from: Cliff Ruggles on June 07, 2019, 03:23:33 AM ---A 1974 Olds is not the aneroid APT model, they were made in 1975 and early 1976.
If that's the case a 1979 Chevy baseplate would physically fit on it, but missing one attachment screw hole (they used 3 in the 1979 and later units).
I don't recommend switching baseplates around on those carburetors. Although they may bolt up there are more often than not differences in them that you may not notice that could make for a less than desirable end result........Cliff
--- End quote ---
Sorry I was confused, it's a 1975 olds 455 quadrajet.
What kind of differences can be expected while switching the base plates? I compared the 79 base with the original 75 and looks almost identical except from the third bolt
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