Carburetor Power Valve; There still seems to be a lot of misconception about Holley carburetors blowing power valves. Nothing could be further from the truth. Holley performance carburetors built 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 percent 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 valves diaphragm can rupture due to an engine backfire! Includes Gasket. Opening Vacuum 4.5 Inch Hg.