Im selling the holley 750 DP off my engine and im going to buy a cheap holley 750 vacuum secondary to rebuild. I see lots of them on craigslist from 50-80 bucks. I understand they are different because of the different chokes, but is there any difference between the carbs themselves? Such as the model numbers are different?

Im considering getting the vacuum secondary, because my double pumper has a bog going WOT at low rpms, plus the vacuum carb might get a few mpg better.

 

If your DP bogs going WOT at low end, wait until you put a vacuum secondary on! The bog is a lean bog that gets covered up by the pump shot. Double pumpers are nice because they give you 2 pump shots. On a vacuum secondary you only get one. I have never had good luck covering up that lean spot using a vacuum secondary carb (on a higher performance engine). The solution is usually to delay the scondary opening (with a heavier spring in the vacuum pod) which means some loss of performance.

 

dont get a vacuum secondary. theyre cheaper because no one wants them. Pancherj is right. tune that double pumper. In all honesty if you have a high performance motor that bog may be due to low power in the bottom end. Change gears to accelerate.