Rebuilding a Holley double-pumper carburetor is a job that rewards patience and the correct rebuild kit. The Holley design is extraordinarily well-supported — every circuit can be adjusted, every component is available as a service part.

The common failure modes: accelerator pump diaphragm, needle and seat, power valve. The power valve is the most important and most commonly incorrectly specified. The Holley power valve opens at a vacuum below its rating. A 6.5 power valve opens when manifold vacuum drops below 6.5 inches — appropriate for a mild street engine. A performance engine with a big camshaft runs less vacuum at idle and needs a lower-rated power valve.

Specify the power valve for your idle vacuum reading, not for the engine's performance level. This single specification affects more driving behavior than any other adjustment in the carburetor.