i have an old rebuilt holley 750 dp # 4779. according to holley it requires 75p and 76s jets with a 8.5 powervalve. why is it that all the 4779-1 thru 4779-8 carbs run 70-71 primary and 80 secondary jets? is there an internal difference in the carbs that require that bid of a jet change? also on a mild street 454 how much of a difference would there possibly be between a hp 950 and the same 750 dp with a proform main body installed? i'm wondering if it would justify the $500 difference. mike

I think the differnces in jet sizes between the models of 750s is related to the powervalve circuit size drilled into the metering block. The two small holes that are fed by the powervalve plus the main jet size determines how much fuel will be flowed through the carb's boosters. You would have to look at the metering block on the other models to determine if the powervalve circuit is larger. I bet Holley lists different primary metering blocks for these differnt models. Most double pumpers flow more fuel through the mainjet and less through the powervalve/enrichment circuit than the more street envisioned designs like the vacume seconday models. Modifications to the metering bocks can be made to the doublepumpers to match the fuel mileage of the vacume secondary carbs and still retain the full performance of the manual secondaries, a good friend of mine has done this many times just by opening up the powervalve channels and jetting down proportionately. If your engine is a mild 454 I'd stick with the 750 and make the mods that may be necessary. JMO