Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor

Idle mixture persision tunning?

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Toronado:
I first tuned it by ear the best I could.
I was not satisfied, so I used a vac. gauge
today to tune the idle mixture screws.
Im not sure about the results i got?
I warmed the engine up blocked wheels
in drive hooked the gauge up it read a bit below
16, so I turned in it went down, and turned them out
and got it to go above 16 , thats the highest i could get it.
Is this the best setting? Is 16 good for 600 rpm?

I don't have a tachometer or i could confirm the rpm more accurately,
and a gas analyzer. i want to get it really tuned this is a 1970
car with 150,000 miles on it, standard vac reading in park with advance
plugged, reads a steady 18 which shows no leaks and lots of life left.

Cliff Ruggles:
I tune them here in a similiar manner, don't use a tach, etc.  The idea is to get the best idle quality at the leanest setting(s). 

If you start out with the mixtures screws backed out about 5 turns, then slowly bring them in until you get  speed change, then back about 1/2 to 3/4 turns, this is usually just about perfect.....Cliff

Toronado:
Your saying a vacuum gauge is not as accurate?
Going by my ear is not so simple with fan noise and exhaust rumbling along,
I can barley hear a jet going over head with my hood up and
my head by the engine running. Thats why I felt the vac. gauge could be a lot more accurate
because numbers cant lie. But what numbers I have read tons of books
on the subject they all say the same thing go for the highest vacuum reading with a steady needle.
My car factory manuel is similar saying 16 inches is factory reading and is best, but no one
seems to be able to explain the details, like can i attain a higher reading by mods/tunning, or is there a best reading and thats that, i have read some new cars idle at 17+ inches?

Toronado:
I did first tune it by ear, but when i did hook the vac gauge up to it
the reading was a steady 15 1/2 inches of vacuum, the i turned the screws watching the gauge.
I was able to turn the screws out all most a full turn each raising the vacuum to over 16 steady inches,
thats better than 15 1/2, but thats my results with this engine and may not be the same for every one. i was basically running a little lean, and after i tuned it with the vac gauge I road tested it & there was a nice improvement from idle to main, it was more snappy and reved higher faster.

Cliff Ruggles:
Actually, most factory literature will describe tuning for a "lean drop", which is usually around 50rpms lower than the highest rpm attainable.

I prefer best idle quality at the leanest setting(s).   This is basically the smoothest idle you can attain backing the mixture screws up about 1/2 to 3/4 turns from when you note a "lean drop" turning them in.

How you get there is not important, by "ear", vacuum gauge, etc.  BTW, a hand held tach works equally as well as a vacuum gauge if you can't hear the exhaust "note" with all the engine compartment noise.......Cliff

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