Author Topic: What does idle mixture screws controls?  (Read 106 times)

Offline BOP455QJ

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What does idle mixture screws controls?
« on: June 05, 2019, 09:06:24 PM »
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 ?
« Last Edit: June 05, 2019, 09:12:07 PM by BOP455QJ »

Offline Cliff Ruggles

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Re: What does idle mixture screws controls?
« Reply #1 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

Offline BOP455QJ

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Re: What does idle mixture screws controls?
« Reply #2 on: June 06, 2019, 07:15:41 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

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

Offline Cliff Ruggles

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Re: What does idle mixture screws controls?
« Reply #3 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

Offline BOP455QJ

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Re: What does idle mixture screws controls?
« Reply #4 on: June 07, 2019, 11:39:46 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

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

Offline Cliff Ruggles

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Re: What does idle mixture screws controls?
« Reply #5 on: June 07, 2019, 12:03:40 PM »
Linkage, available vacuum ports, PCV, partial ported vacuum for the EGR, transfer slots length and location, idle bypass air (size and location), holes under the mixture screws, thread pitch of the mixture screws, etc.

I try to deter folks from swapping them around unless you verify that EVERTHING is identical in every respect.  Much better to find a donor carb with the same number, although many "runs" of similar numbers will produce suitable donor parts.  For example find another Old's baseplate from a 75-78 model and at least you'll be closer than grabbing one from a 75-78 Chevy automotive carb (the trucks were different)......Cliff