Topic: 1980 Quadrajet
in Forum: C3 Fuel, Emission Control, and Exhaust Systems
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What's a good setting for the idle mixture screws on this carb. Bear in mind there is no smog equipment left on it, and it has a K&N air filter on it. I'm around 2500 ft above sea level. Got them set 1and1/2 turns out right now seems a little rich. Thanks Ric.
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1 1/2 turns out should be about right. I usually set them at 2 rounds out when building them, and that gets them up and running.
I'd suggest hooking up a vacuum gauge, and tuning for the highest vac. reading. Remember to "clear it out" after each adjustment, as they can load up a bit while idling, if the mixture is too rich.
HTH
I'd suggest hooking up a vacuum gauge, and tuning for the highest vac. reading. Remember to "clear it out" after each adjustment, as they can load up a bit while idling, if the mixture is too rich.
HTH
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There is no one "magic setting". Idle enrichment adjustments are provided because it is a very precise adjustment that can't be manufactured into a carburetor like main jets. You'd be suprised at the difference between the two sides of some carburetors.
Here's the adjustment procedure I've used for years:
1) Pre-set the jets to 2 turns out.
2) Warm up the engine
3) Set the idle to 550 RPM
4) Verify timing is right
5) Slowly turn one jet in until you just hear a change in the idle - can also be verified with a vacuum gauge (vacuum drop)
6) Slowly turn the same jet out until you just hear a change in the idle, counting the turns as you go - can also be verified with a vacuum gauge (vacuum drop)
7) Turn the screw back exactly half the turns you counted (ie. If the count is 2.5, turn back 1.25 turns)
8) Do the same with the other idle mixture screw
9) Check your idle speed - if idle has increased, repeat steps 3 - 8.
Note: You need to do his at least 4 times / year.
Turn the screws all of the way in, and turn them out the same amount as a starting point, making sure they are even.
Start the car and let it idle at normal speed.
Connect a tach and vacuum gauge.
Slowly turn the screws in one at a time until the idle speed and vacuum starts to drop.
Turn it back out until the rpm and vacuum peaks.
Do this with both screws. Keep them even.
You will find a spot where they do not respond, neither speeding or slowing the engine with very little vacuum change.
Turn them as far in as you can without going past this point, and dropping speed or vacuum.
If the screws are uneven, there is something else wrong. Either the carb needs rebulilt, there is a vacuum leak, poor compression, or something not right with the engine. They should balance within about 1/4 turn or less.
Peaking them and then running them in is the recommend way for emissions, but it is NOT where they run the best.
If you have a good ear and listen close, you can actually hear the air flow through the carb. But I don't recommend trying to set it by ear without a LOT of practice. Starting the way I suggested, the finishing by ear I have set them as well as could be done with an exhaust gas analyzer. But that takes a lot of practice.
Make sure the timing is set and all other adjustment are made first. Be sure the PCV is working properly and the EGR is fully closed.
If the engine RPM comes out of spec, readjust it to the correct idle and repeat the procedure.
You will find as you go through altitude changes, the adjustment should be tweaked to compensate.
Ken, will this be the same for a 1980 SMOG EQUIPED car such as in California ?
Yes it will. But when you go to get the smog test done, turn each screw in equally to get a 50 RPM drop total. This could also be stated as about 1/2 turn in each.
The factory procedure was to add a small amount of propane, then peak the carb, then when the propane was removed it was okay. But this was with a preset orifice supplied as a required GM tool. The 1/2 turn will get you there.
Thanks Ken !
Ken,
Great advice. Worked GREAT!!!
Great advice. Worked GREAT!!!
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