There still seems to be a lot of misconception about Holley carburetors
blowing power valves. Nothing could be further from the truth, according to the company. Holley
performance carburetors since 1992 have utilized a power valve check
system that effectively eliminated this infrequent problem. Consisting
of a spring, brass seat and check ball, the check ball system is 100%
effective protecting the power valve diaphragm from damage due to
engine backfire.
The power valve check ball is designed to be normally open but quickly
seals to close off the internal vacuum passage when a backfire occurs.
Once closed, the check valve interrupts the pressure wave generated by
the backfire, thus protecting the power valve diaphragm. There is no
way that the power valve’s diaphragm can rupture due to an engine
backfire, according to the company.
Holley’s Power Enrichment System
Holley carburetors utilize
a vacuum operated power enrichment system and a selection of power
valves is available to “time” this system’s operation to your specific
needs. Each Holley power valve is stamped with a number to indicate the
vacuum opening point. For example, the number “65” indicates that the
power valve will open when the engine vacuum drops to 6.5? or below.
An accurate vacuum gauge, such as Holley P/N 26-501, should be used
when determining the correct power valve to use. A competition or race
engine which has a long duration high overlap camshaft will have low
manifold vacuum at idle speeds. If the vehicle has a manual
transmission, take the vacuum reading with the engine thoroughly warmed
up and at idle.
If the vehicle is equipped with an automatic transmission, take the
vacuum reading with the engine thoroughly warmed up and idling in gear.
In either case, the power valve selected should be 1/2 the intake
manifold vacuum reading taken. EXAMPLE: 13? Hg vacuum reading divided
by 2 = 6.5 power valve. If your reading divided by 2 lands on an even
number you should select the next lowest power valve. EXAMPLE: 8? Hg
vacuum reading divided by 2 = 4 power valve. Since there is no #4 power
valve you should use a 3.5.
Most of the popular Holley “Street Legal” and “Street Performance”
carburetors incorporate a power valve blow-out protection system. A
special check valve is located in the throttle body expressly for this
purpose. This check valve is designed to be normally open but will
quickly seat to close off the internal vacuum passage when a backfire
occurs. Once closed, the check valve interrupts the pressure wave
caused by the backfire, thus protecting the power valve.
If you have a carburetor older than 1992 (or you have experienced an
extreme backfire) and expect a blown power valve, use this simple test.
TEST: At idle turn your idle mixture screws (found on the side of the
metering block) all the way in. If your engine dies the power valve is
not blown.
– Tech Tip courtesy of Holley Performance
