Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor

Idle mixture persision tunning?

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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

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