TECH INSTALL FABRICATE UPGRADE
linkage and the actual diaphragm.
If clearance exists, merely adjust the
pump linkage with a pair of^3 ⁄ 8 -inch
wrenches. Once this is properly adjusted,
it’s also possible that the accelerator
pump nozzle size can be reduced. Too
much fuel from an overzealous tuning
approach can be just as detrimental to
good throttle response and too little
nozzle size, although the driveability
symptoms will be different.
- Convert That
4160 to a 4150
So you bought a newer 0-3310
vacuum secondary 750 Holley for a
great price, but later you realize it is the
4160 style with the secondary metering
plate. These are still great carburetors
but you’d rather have the more race-
inspired look of the 4150-style Holley
with the secondary metering blocks. No
sweat—there’s a conversion kit for that.
Holley makes several kits—mainly
for either the 0-3310 750-cfm or the
0-1850 600-cfm carbs. The conversion
kit adds the rear metering block along
with the required different metering
block gasket. The kit also comes with
the correct jets to retain the stock
jetting. For example, the 0-3310-5 4160
750 carburetor comes stock with a PN
134-21 secondary metering plate that is
equivalent to a 75 jet that is included in
the kit.
Dual inlet fuel bowls are not
included in the kit but you can add
them at the same time. This can get a
little expensive and it might be cheaper
just to buy an older Holley just to obtain
the parts. Of course, you can also just
convert the carb to a 4150 style with
the metering block and retain the stock
single-inlet configuration.- Traveling Down the Slot
Here’s where a little knowledge
can make you a master carb tuner.
A continuing problem for street
engines running big camshafts and
lots of overlap is low idle vacuum.
With minimal vacuum, these engines
demand increased throttle opening in
order to idle at an acceptable speed.
Unfortunately, when the curb idle
adjustment is opened past the stock
setting, the throttle blades uncover the
transfer slots. These slots are intended to
pull additional fuel from the carburetor’s
idle circuit that is controlled by the idle
feed restrictor. The transfer slot adds
fuel to compensate for increased throttle
opening before the main metering
system kicks in. This prevents a stumble
under light acceleration.
When the throttle blades are opened
far enough at idle to uncover the
transfer slots, this adds idle fuel that
is not controlled by the idle mixture
screws. This increases fuel flow making
the idle air/fuel ratio extremely rich. Yet
when additional throttle is added for
light acceleration, the engine stumbles.
The traditional solution (which
works very well) is to close the curb
idle back to where it just touches the
transfer slot and then drill a small initial
hole; usually^3 ⁄ 32 inch (0.093-inch) in
each primary throttle blade. If the idle
speed is still too low, larger holes or
two more in the secondary throttle
blades can be drilled. It’s also possible to
slightly increase the secondary throttle
stop to slightly increase airflow but
again only enough to avoid opening
into the secondary transfer slot. The
ideal placement of the closed primary
and second throttles should be with
the blades uncovering barely 0.030- to
0.040-inch of the transfer slot.
The only problem with drilling
these holes in the throttle blades is
that this becomes a semi-permanent
modification. If the carb will be
subsequently used on a milder-cammed
engine, it will probably not be possible
to return the curb idle speed low enough
because of the permanent holes. This
can be repaired but will require new
throttle blades.
When Holley redesigned the Ultra✦ The conversion kit (foreground) replaces the metering plate like on the 416 0-style carb
on the left and replaces it with a metering block like the 415 0-style carburetor on the right.
In case you’re taking notes, the secondary metering block is the main difference between
4160 and 415 0 carburetors.
✦ This shows the curb idle set with a large
exposed portion of the transfer slot—this is
bad and will cause not only an overly rich
idle mixture but also an off-idle stumble.✦ In a properly adjusted curb idle setting,
the tip of the throttle blade should just
uncover the end of the transfer slot (as
shown here).48 classictrucks.com