I know, I know this has been covered a thousand times.....just indulge me and make it a thousand and one times.

Question.....when both A/F screws are bottomed out on a '79 Rochester Quadrajet, auto trans stock L82.....how many FULL TURNS are made when backing the screws out, as just a rough setting and starting point?

Question....I no longer have a vacuum gauge......however when I used to adjust Carter AFB carbs, the manual procedure was relatively simple.......initially turn each A/F screw 2 1/2 turns out, trans in neutral, engine idling at or around 750 to 850 rpm, turn one A/F screw in until rpms are heard to drop a bit, then back the screw out a half turn. Same with the other screw. Reset idle to preferred speed, and drive off....usually close enough for "normal" driving.

Will this procedure work on a Quad? as I am having much difficulty in locating someone QUALIFIED to make these adjustments. As it stands now (after having had a kit installed and set by some kid that couldn't have been a day over 22, even though he was SAID to be an expert on old Vettes), the engine will NOT hold an idle while in gear, like stopped at a stoplite....gradually dropping down in rpms and nearly stalling...probably would've had I not gently pushed on the "go pedal" a bit.

Raising the idle speed did little to correct that problem except for raising the idle speed when in NEUTRAL. As soon as the tranny is put in drive, the rpms continue to drop and would surely stall if allowed to continue.
I discovered what is referred to as a fast idle solenoid on the side of the carb, which was NOT adjusted properly at ALL when the A/C was on....I remedied that quickly enough...but THIS problem is with the A/C off, engine warmed up.

It had run FINE but it had a bad stumble on accelerating, and I ASSUMED the accelerator pump was bad...and THAT is why the kid installed the kit. The stumble is 95% gone, but now as mentioned, the engine refuses to idle when in gear stopped at a traffic lite.
Sorry for the long drawn out explanation.....just trying to be thorough.....

 

Sounds like the A/F ratios a tad lean at idle? Maybe screw the mixture screws in (counting the turns) to see where the "kid" has them and then turn them out an extra 1/4 turn from his setting and see what happens.

 

I think 2 turns out is the magic number, but it’s only magic, not science.

My quad needed 4-5 turns out IIRC. Depends on the engine, the idle fuel tube sizes, how large the passengers behind the idle screws are, etc.

 

The '79 M4M carbs can have idle mixture screw settings anywhere from 2 to 8 turns out - there is no magic number. When I set up an M4M carb for initial testing, I start with the screws at 6 turns.

Here is my suggested procedure for setting up idle mixtures on the Q-Jet:

Once the screws have been set to an initial starting position (4 turns out on 4MV carbs and 6 turns out on M4M), start the engine and let it warm up. Keep in mind: Turning the idle mixture screws IN will lean it out. Backing them OUT will richen the idle mixture. Adjust the idle mixture screws evenly to obtain best quality idle. If you have a little bit of an ear and a feel for the car, you don’t need a vacuum gauge to do this: just listen and feel for what the engine wants. Back both screws out a turn and “feel” if the engine idle quality improves. If it does, run them out another turn. If it doesn’t improve or change, run the screws in instead. Observe if idle quality improves or degrades – you’re on a “fishing trip” to find the sweet spot. Always adjust both screws the same – you cannot adjust one screw until best idle is achieved, and then adjust the other screw – that’s a sure way to get things “screwed up.” Once best quality idle is achieved, run the screws as lean (inward) as possible while maintaining that best idle quality: the moment idle quality starts to degrade, back them out ¼ turn to assure you’re in the “quality idle” zone without being too rich. If you need to pass an emissions test, run the screws in (lean) to obtain a 50rpm idle speed drop after best idle quality is achieved: This is as outlined in the GM Service Manual, and is referred to as “lean drop” idle mixture setting. Set idle speed, and you’re ready to go!

As a side note, there are no 22 year old Corvette "experts." If the guy "put a kit" in the carb, he also has the float level messed up, and the APT is adjusted wrong. These factors will affect fuel flow at idle and can cause the carb to never respond correctly to idle mixture screw adjustments.


Lars

 

Lars, why do you think my 1971 base 350 idles fine at 1000:rpm and then stalls when I turn below 1000. I have had some engine missing, back firing, and stalling issues the last several months. I have owned the car about 15 years and 15,000 miles and it about always ran great until recently.

So far I have timed and dwell many times, change air filter, fuel filter, condenser, points, rotor, tried to clean carb with carb cleaner. Thank you

 

Please dont resurrect a 4 year old thread with a new question. Start a new thread, please.