✦ The conventional fix for big-cammed
carbureted engines is to drill holes in the
primary and sometimes in the secondary
throttle blades near the transfer slot with
sufficient size to allow closing the curb idle
adjustment back to its ideal location.
✦ This is the idle stop adjustment for most
Holley carburetors. To access the carb must
be inverted. Make very slight adjustments
here as the factory setting is very close to
ideal.
✦ This Holley Ultra XP and some older
HPs employ a bypass idle circuit with an
adjuster screw located underneath the
air cleaner stud mounting boss. You can
identify these carbs by the four vertical
holes adjacent to the air cleaner stud that
allows air (controlled by the adjustment
screw) to bypass air past the throttle
blades.
XP line of carburetors, they added an
adjustable air bypass feature located
underneath the air cleaner stud. With
this carburetor, idle speed adjustments
should be done with the bypass
adjustment rather than at the throttle
speed screw on the linkage.- Vacuum Secondary Tricks
If you’ve ever tried to change the
vacuum secondary spring in a Holley
diaphragm module, you know it requires
dexterity and patience. Sean Murphy
showed us how to do it without tearing
the fragile rubber. Place the rubber
diaphragm into the lower housing and
then set the base on your vise with the
steel shaft extending through the jaws.
Position the rubber diaphragm on the
base in the correct orientation. Make
sure the hole that seals the diaphragm
transfer slot is uncovered. Lightly clamp
the steel diaphragm shaft in the vise.
Now shoot a little WD-40 or other
lube on the threads of the four screws.
Place the vacuum secondary spring in
the boss on the underside of the lid.
For most street applications, Murphy
likes to use the purple spring. With the
diaphragm held in place by the vise,
lightly compress the spring over the
diaphragm and align the holes between
the base and the cover. Start each screw
by hand as the lid is held in place on
the diaphragm. Be careful here to not
catch the rubber with the threads of the
screws.
Murphy also prefers to eliminate
the stock steel ball that Holley uses to
trim the opening rate. Instead he drives
in a lead ball and drills it to 0.040 inch
to act as a restrictor so the secondary
opens at a slow enough rate to prevent
a bog or hesitation.- TPS Conversion
An increasingly popular swap is
to use the electronically controlled
4L60E or 4L80E automatic overdrive
transmissions in older muscle cars. One
of the requirements for this conversion
is it requires a throttle position sensor
✦ If you like experimenting with vacuum diaphragm springs, the best route is to use a
quick-change lid. This makes swapping springs really quick and easy.✦ Here’s the trick to assembling that
tricky vacuum diaphragm canister. Place
the base on top of the vise and slide the
diaphragm through the lower half until it
sits flush. Clamp the shaft in the vise and
then carefully lower the lid in place. Start
the screws lightly to avoid winding up the
diaphragm, which could tear the rubber.November 2019 classictrucks 49