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Old 04-24-2006, 02:11 PM   #1
atomickustom
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Default 1978 Quadrajet idle mixture adjustment?

The stock Quadrajet on my 1978 350 has no idle mixture adjustment screws! I didn't notice this until I'd already gone to the trouble of cleaning the carb, bolting it onto my new manifold, and adapting all my linkages, etc. It runs fine, but I'm sure I could smooth out the idle a bit more if I could rich it up just a teeny bit.
There are just silver metal plates filling the round holes where the screws would normally be. Are these just covers? If I pry them off, will I destroy my carb, or will I find adjustment screws patiently waiting for me to adjust them?

I DID try to search to see if this has already been covered. If it has, please just post a link to the old discussion.
Also, I know that a 1978 Quadrajet isn't exactly "traditional" and don't need anyone to remind me. The entire running gear in my '53 Chevy is from a donor car (1978 Impala wagon = CHEAP), and I'll take "cheap" over "traditional" any day. You wouldn't believe how many parts you can use off of one good donor car!
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Old 04-24-2006, 02:25 PM   #2
OLDSKEWL61
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Default Re: 1978 Quadrajet idle mixture adjustment?

There Are Just Covers I Ussally Jusy Drill A 1/8 In Hole From The Bottem Up Angle Toward The Front And Stop Before You Go Through All The Way And Drive Out The Cap With A Punch.

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Old 04-24-2006, 02:52 PM   #3
HEATHEN
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Default Re: 1978 Quadrajet idle mixture adjustment?

Quote:
Originally Posted by atomickustom
The stock Quadrajet on my 1978 350 has no idle mixture adjustment screws! I didn't notice this until I'd already gone to the trouble of cleaning the carb, bolting it onto my new manifold, and adapting all my linkages, etc. It runs fine, but I'm sure I could smooth out the idle a bit more if I could rich it up just a teeny bit.
There are just silver metal plates filling the round holes where the screws would normally be. Are these just covers? If I pry them off, will I destroy my carb, or will I find adjustment screws patiently waiting for me to adjust them?

I DID try to search to see if this has already been covered. If it has, please just post a link to the old discussion.
Also, I know that a 1978 Quadrajet isn't exactly "traditional" and don't need anyone to remind me. The entire running gear in my '53 Chevy is from a donor car (1978 Impala wagon = CHEAP), and I'll take "cheap" over "traditional" any day. You wouldn't believe how many parts you can use off of one good donor car!
25 years ago, all of the parts stores sold "modification kits" that came with a hole saw to cut out the plugs, and a special adjusting tool, since the ends of the mixture screws resemble a clutch head, like '40s and '50s Chevies are held together with. Thexton made such a kit, but I don't know if they still do.
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Old 04-24-2006, 03:06 PM   #4
Flatman
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Default Re: 1978 Quadrajet idle mixture adjustment?

You might be able to grab the screw head with some small needle nose pliers, once you get the covers off
Those Q Jets were great carbs, huge secondaries

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Old 04-24-2006, 03:34 PM   #5
53sled
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Default Re: 1978 Quadrajet idle mixture adjustment?

Lisle tool and maybe Motormite sell q-jet tools with the funky end. Drill a small hole, run a screw in, pop out.
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Old 04-25-2006, 11:19 AM   #6
atomickustom
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Default Re: 1978 Quadrajet idle mixture adjustment?

Thanks for the info, everyone. I'll be drilling and popping soon.
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Old 04-25-2006, 04:11 PM   #7
bigdog
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Default Re: 1978 Quadrajet idle mixture adjustment?

Those mixture screw plugs might be hardened steel- they were on the one I just did. Drill bit won't touch 'em if they are. I went and dug out the instructions for removing the plugs.

1.invert throttle body
2. place punch between two location marks over idle mixture plug
3. break out throttle body to provide access to the hardened steel plug, drive plug out

4.late models-cut 2 parallel slots on either side of the location marks using a hacksaw. slots should not exrend beyond 1/8 inch past the marks
5. position a flat punch at a 45 degree angle between the ends of the saw marks in the throttle body, drive the punch between the slots causing the slug to break off.
6. hold a center punch in a vertical position and drive it into the steel plug, then angle the punch 45 degrees and drive the plug out.

Fun, huh? always loved beating on carburetor with hammers.
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