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I'm replacing my worn holley 750 double pumper, i'd like to get a 650cfm HP style Holley. Its a T/A w/ a 6.6l, auto, and stock ratio. Can anyone recommend a secondary type? Mechanical or Vacuum. The old carb worked ecellent when it was new...
 

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Overall an automatic tranny in a street car works best with vacuum secondaries. This is because, unlike manuals, auto's don't have instantous high loads from a dead stop or while driving. If you ever take the car to the track you will notice it takes too long for the vacuum secondaries to open unlike a double pumper. Some say the only way to fix this is with different spring combinations for the vacuum secondaries. But the best solution that worked for me was putting a small screw and nut in the slot where a rod from the primaries pulls back the secondaries if they don't themselves. Even with a stock primary accel. pump and obiviously none for the secondaries I never had a stumble.

 

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I really liked the vacuum secondaries on my Rochester. Good gas mileage if you keep your foot out of it, and pretty good power/response, when you need it. Overall a good comprimise
 

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Holley used to offer a kit to adjust the vac secondary. It had a number of springs that allowed you to adjust the opening rate of the secondaries. If you do go with the vac sec carb see if this kit is still available.
 
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