Some high overlap cams idle at 5" but cruise at 15" and need 8.5" power valves. 2.56 gears need to transition sooner than 5.13 gears. A heavy vehicle needs to transition sooner than a light vehicle. What does that have to do with the power valve? You need to find the point where the motor needs to transition from the 'cruise' mode mixture to the 'power' mode mixture. I can make my idle vacuum change 5" just by turning the idle speed screw. Stock engines can have high vacuum readings (10-18 inches at idle) and the Holley power valves with higher readings like 6.5 to 10.5 will work correctly. When the motor leans out (you'll feel it surging at that point even if you don't have an air/fuel meter) look at the vacuum gage and pick a power valve 1-2" higher. Each kit contains: Gaskets Pump diaphragms Needle(s) and seat(s) Power valve float gauges Detailed instructions Find Holley Carburetor Rebuild. Estimated to ship direct from manufacturer on 10/13/22, pending manufacturer availability. Go up a hill in high gear (to put a load on the motor) and hold the vacuum at 8" and watch the air/fuel meter. Holley Single-Stage Standard Flow Power Valve Opening Vacuum: 9.5' Hg. Put in a really low power valve like a 3.5". I thought, perhaps the power valve was blown and its getting extra fuel. If you have an air/fuel meter and a vacuum gage you can find the exact valve needed. A 750 double pumper Holley carburetor is a high performance carburetor that. I can't believe Holley would spread a myth like "half the vacuum at idle" for a power valve.
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