There still seems to be a lot of misconception about Holley carburetorsblowing power valves. Nothing could be further from the truth, according to the company. Holleyperformance carburetors since 1992 have utilized a power valve checksystem that effectively eliminated this infrequent problem. Consistingof a spring, brass seat and check ball, the check ball system is 100%effective protecting the power valve diaphragm from damage due toengine backfire.
The power valve check ball is designed to be normally open but quicklyseals to close off the internal vacuum passage when a backfire occurs.Once closed, the check valve interrupts the pressure wave generated bythe backfire, thus protecting the power valve diaphragm. There is noway that the power valve’s diaphragm can rupture due to an enginebackfire, according to the company.
Holley’s Power Enrichment System
Holley carburetors utilizea vacuum operated power enrichment system and a selection of powervalves is available to “time” this system’s operation to your specificneeds. Each Holley power valve is stamped with a number to indicate thevacuum opening point. For example, the number “65” indicates that thepower 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 usedwhen determining the correct power valve to use. A competition or raceengine which has a long duration high overlap camshaft will have lowmanifold vacuum at idle speeds. If the vehicle has a manualtransmission, take the vacuum reading with the engine thoroughly warmedup and at idle.
If the vehicle is equipped with an automatic transmission, take thevacuum reading with the engine thoroughly warmed up and idling in gear.In either case, the power valve selected should be 1/2 the intakemanifold vacuum reading taken. EXAMPLE: 13? Hg vacuum reading dividedby 2 = 6.5 power valve. If your reading divided by 2 lands on an evennumber you should select the next lowest power valve. EXAMPLE: 8? Hgvacuum reading divided by 2 = 4 power valve. Since there is no #4 powervalve 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. Aspecial check valve is located in the throttle body expressly for thispurpose. This check valve is designed to be normally open but willquickly seat to close off the internal vacuum passage when a backfireoccurs. Once closed, the check valve interrupts the pressure wavecaused by the backfire, thus protecting the power valve.
If you have a carburetor older than 1992 (or you have experienced anextreme backfire) and expect a blown power valve, use this simple test.TEST: At idle turn your idle mixture screws (found on the side of themetering block) all the way in. If your engine dies the power valve isnot blown.
– Tech Tip courtesy of Holley Performance