Idle mixture persision tunning?

[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

[Schurkey]

On this, I'm going to disagree with Cliff.

1.  A vacuum gauge is MUCH more sensitive than (and therefore preferable to) a tach for idle mixture tuning.

2.  There is a TREMENDOUS amount of emission reduction between "best idle" and a slight "Lean drop".  Turning the idle mixture screws just a touch lean will not affect idle quality much, but it makes a BIG difference in the amount of pollution coming down the exhaust system.  A car with a catalytic converter won't like "best idle" as the cat will run hot.  A car without a catalytic converter will just spew a LOT more hydrocarbon emissions.  Any time I can maintain 99% of the idle quality and reduce pollution--I do.




As for the absolute reading--15 inches of vacuum vs. 16 vs. 18 or whatever--every engine is different.  What's important is that the reading should be STEADY, not that it achieve any absolute value--although "more" is "better".  Cam timing, ignition timing, engine wear, fuel/air mixture ratio--all that and more play into the idle vacuum reading.

[omaha]

my 350 pontiac measures 18 on vacuum. quite alot I think for a 1970 engine with a lot of wear on it.
It makes too much for the carb on it now. wont let the power piston rise quick enough. I've been meaning to go through it. ... how are the springs rated in the kits?(for vac reading) on the 4 springs

[Toronado]

This information is very helpful in understanding
the complex approach to tunning a carbs idle mixture,
I hate emissions equipment because it often does nothing
for performance and often hinders it.  At 16 I might be a tad
rich, but i dought it, the highest vac reading is still a steady needle,
by the science approach you want the highest vac reading"these carbs are vacuum operated"
while maintaining a steady needle, its when the needle goes up and down or flutters when the mixture is off according to science. I could probably run it just fine at 15 hg but i figure the engine depending
on temp, moister, heat and most of all old gaskets that can leak at any time it should
be ran at highest vac, at least I believe that now. You cant hear any difference at 16 or 15,
but it did feel more responsive from idle to WOT.

[Cliff Ruggles]

Best idle quality at the leanest setting(s).  Pretty much covered that in my first two responses.

Keep in mind that folks tend to associate "lean" with "efficient", like it's going to run better and get better fuel economy, etc. 

Lean fuel mixtures are incredibly difficult to burn, and a LOT of noxious pollutants will exit your tailpipes if you venture toward lean settings for any particular combination(s) of parts.

Most vehicles these carburetors are used on will NOT have all of the factory emission equipment in place and operational. 

If we set up 100 carburetors in the shop for these older vehicles, at most there will be one or two that are still "stock".  Folks change a LOT of parts out, in search of improved performance.  The catalytic converters get removed, factory intakes gone, EGR plugged off, cooler thermostats, blocked exhaust crossovers, air pumps amputated, aftermarket intakes installed, "high performance camshaft, etc, etc.

In any and all cases, when it comes to setting the carburetor idle mixture screws, set them so the engine idles the smoothest and is happy for the combination of parts being used.  This usually ends up at or very close to "lean drop" for most applications.   Many heavily cammed engines will not like being anywhere near "lean" at idle, and tend to like a lot of timing and a lot of  fuel at idle to be happy.  The closer the engine is to "stock, the happier it will be with leaner idle fuel setting(s), as a general rule......Cliff