TECH INSTALL FABRICATE UPGRADE
Holley carburetor romance has to do
with an old-school look that includes
that retro zinc chromate appearance.
But over time, the carburetors lose
that nice, shiny appearance and end
up faded and looking a bit haggard.
But the good news is that original
appearance is easy to restore. While
it is possible to do this yourself, it
involves buying chemicals and a
knowledge of the technique. That’s
probably worthwhile if you are going
into business, but otherwise it’s easier
just to ship your carb to SMI and
have him recolor your cab quickly
and easily. Of course, this is also the
perfect time for perhaps an upgrade or
two to make the carb work as good as
it will now look.
- Power Valve Tuning
There’s more to tuning the Holley
power valve on the primary side than
just choosing when the valve opens.
Let’s say you have a 750 double-
pumper Holley 0-4779. On your
engine, let’s say someone (not you, of
course) has messed with the carburetor
and it now has 75 primary jet and an
85 power valve (71 jet is stock). This
means the power valve will open at
8.5 inches of vacuum. On all Holleys
there are two power valve channel
restrictors (PVCR) that determine
the amount of fuel delivered to
the boosters over and above the
fuel metered by the jets when the
power valve opens. Generally these
restrictors are permanently drilled
passages in the metering block. The
newer HP and Ultra XP carbs come
with screw-in restrictors.
There are several tricks you can try
to improve part-throttle performance.
The first would be to lower the power
valve opening point to 65 (which is
stock at 6½ inches Hg) that would
open the power valve later when
the engine vacuum is lower, with a
greater load. If you think that the
jetting might be a bit rich at light
throttle, you can try running leaner
primary jets. Reducing the primary
jets will lean the WOT jetting so you
would need to compensate. One way
to do that is to increase the size of
the power valve channel restrictors.
So let’s say we want to reduce our
primary jetting from 75 down to a 70
and the PVCRs are currently sized at
0.060 inch.
We need to determine the areas
of the jets and the power valve
restrictors. We’ll save you the grief of
slogging through the math. Reducing
the main jets from 75 to 70 is equal to
a 13 percent reduction in flow area.
This means we have to increase the
PVCR by the same area, which came
out to very close to a 0.062-inch
restrictor. At WOT when the power
valve opens, the overall fuel flowwould be nearly identical, but with
this combination the part-throttle air/
fuel ratio (before the power valve
opens) would be leaner by four
jet sizes. Of course, if the opposite
combination is what the engine
needed, you could easily modify the
PVCR system to tune in that direction
as well. It’s best to start with the stock
jet and PVCR combination first before
you start tuning but this is a way to
dial in the carburetion for what your
engine really needs.PARTS LIST
Description PN SourceHolley Viton acc. pump diaphragm, green 135 -10 Summit RacingHolley conversion kit 416 0 to 415 0 (0- 33 10) 34 -1 3 Summit RacingHolley conversion kit 416 0 to 415 0 (0-18 5 0) 34 - 6 Summit RacingHolley diaphragm quick-change lid 20- 59 Summit RacingHolley vacuum secondary spring kit 20-1 3 Summit RacingHolley TPS conversion kit, 416 0- 4150 534 -202 Summit RacingHolley TPS conversion for Dominators 534 -21 4 Summit RacingInnovate TPS adapter 3930 Summit RacingHGM AccuLink TPS kit AccuLink HGM Electronics✦ A set of calibrated pin vises really helps dial in restrictor sizes. These kits are inexpensive
and are incredibly helpful when tuning PVCRs. In this billet metering block, the PVCR
measures 0.0 5 1 inch in diameter. Some blocks come with screw-in PVCR—this block just
uses a drilled passage.54 classictrucks.com