leave some information on the table.
But digest each of these and they
will generate the energy to get you
started on the path to pure Holley
comprehension. The knowledge base
for Holley tuning is not likely to
become a lost art anytime soon, and if
you become proficient, you just might
become the neighborhood expert.
- Set Idle Correctly
This might seem way too simple,
but for a carbureted street engine this
is the most important tuning step you
can make because street engines spend
a majority of their time at or just above
idle. The first step is to obtain a low-
speed tach that will show minor changes
in rpm along with a vacuum gauge. For
this procedure we’ll assume a Holley
with only two idle mixture screws.
Many performance carbs now offer four
screws but the procedure would still be
the same.
Before the engine starts, check the
position of the idle mixture screws
(located on either side of the metering
block) by counting the turns on each
one in until it lightly seats. Let’s say that
the left side is three-quarter turn out
and the right is one-and-a-half turns
out. The best route here is to split the
difference and make them both one turn
out.
With the engine idling at normalized
temperature, try leaning the mixture
with a minor clockwise turn of both idle
mixture screws. Make these changes
very slight and note the effect on the
engine. If the idle speed and vacuum
increase, continue with similar changes
in small increments until the idle speed
or vacuum deteriorates. If the initial
change results in a loss of rpm or
vacuum go back to the baseline and
then turn the idle mixture screws in the
opposite or counterclockwise direction.
With each small change, wait for a
moment to allow the engine to stabilize.
This is where the vacuum gauge and
tach can really help. Make changes that
stabilize the vacuum gauge needle. If
idle speed increases, lower it by closing
the idle speed adjustment. An air/fuel
ratio meter can also help, but don’t fall
into the trap of shooting for a given
number. Set the idle mixture for the best
idle quality along with the leanest air/
fuel ratio. If the engine will idle at 14:1
air/fuel ratio that’s really good. Most
performance engines will need to be
richer—usually around 13.5:1. There is
a tip-over point between rich and lean
where combustion is ideal. This is near a
lean mixture as long as the idle is stable.
Cars with automatic transmissions
may need a slightly richer idle mixture
setting to allow the engine to idle✦ Accurate tuning of the idle mixture with a combination of a vacuum gauge and sensitive
tachometer can offer excellent results—and the best part is these tuning efforts cost
nothing to achieve.smoothly in gear. For tight converters,
we’ve had to add a half richer mixture
to keep the engine running in gear.
Much of this is determined by the
tightness of the converter.- Accelerator Pump Linkage
Adjustment
If your engine has an annoying
stumble just off idle that you can’t
seem to fix, check the accelerator pump
linkage. Lightly open the throttle linkage
and watch the accelerator pump squirter
in the primary venturi. The moment the
throttle linkage moves, fuel should exit
the nozzle. If it does not, look first for
slack or clearance between the vertical
accelerator pump linkage arm and the
lever that moves the actual accelerator
pump diaphragm.
There is some confusion around
this as older tuning stories mention
requiring clearance here, but only with
the linkage at wide-open throttle (WOT).
At idle, there should be zero clearance
or even a slight preload between the
✦ Adjust the linkage on the accelerator
pump so that the instant the throttle moves
the accelerator nozzle will squirt fuel. If it
doesn’t, the linkage is improperly set. There
should be zero clearance or a slight preload
on the linkage arm to the accelerator pump
lever.✦ We’ve seen the standard accelerator
pump diaphragm become brittle with
age. Holley sells a green Viton rubber
diaphragm intended for alcohol carbs but
works great with the 10 percent ethanol in
today’s pump gas.November 2019 classictrucks 47