# Holley 750 Double Pumper or Vacum secondary???



## deze2000 (Aug 28, 2009)

I have a 1969 Pontiac Custom S between a GTO and a Tempest but this car has a 389 bored over 30 hedmen headers 2.5" out the back. We are going to swap the TH-350 for a stage-1 performance 200-R4 that has an over drive gear and upgrade the 2.89 rear gear to a 3.73 Auburn posi unit. So I'm looking to get either a 750 double pumper or a Vacuum 750. Since the car has an overdrive the gas milage is a not issue. What I understand is that the double pumper gives a quicker throttle response. Currently the car has a holley replacement 650 spread bore vacum and runs almost but not quit lean. I will do both with the car street, cruising and track. 90-10 90% street and 10% street. Let me know what ya'll think. We have a stock cam and has Pontiac 48 72cc heads and pontiac intake and are looking to add a mild higher mid range cam later on. So with that said let me know.


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## Silver69GTO (May 25, 2009)

A 750 Double pumper will be too much for your engine. You'll get a serious bog off the line.
I also think any 750cfm carb will run the engine too rich.
Most aftermarket Holley's are square bore carbs so if your intake is a spread bore you'll need to get an adapter.
Stick with the 650 spread bore Holly but fine tune it.
Get a jet kit and a secondary spring kit for it and start experimenting.
If the carb mixture (rich/lean) is perfect now, then the jet kit is not needed.
Play with the pump shot and the secondary springs.

Factory cfm ratings of the Q-jet carbs weren't true most of the time. A factory 750cfm Q-jet usually flowed considerably less.
Holley's are usually right on the money.
My '69 GTO has the correct '69 400 in it but someone had swapped the original intake and Q-jet for an Offey intake/650 vacuum secondary Holly combo.
I tried a 750cfm vacuum secondary Holley but it had a bad bog off the line and it ran too rich at all RPMs.
When I first got the car,it had a bad lifter tick when first starting it for the day. While I was installing a new 068 cam and lifters, I installed an original cast iron intake and Q-jet.
The GTO didn't seem to feel as snappy as before so I decided to try the Offey/Holley combo with the new cam.
That combo seemed to wake it up.
I ended up with the lightest Holley secondary spring with the Holley factory set pump shot.
Anyway, you get my drift. Play with the 650 first before you go bigger and possibly the wrong direction.


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## Too Many Projects (Nov 15, 2008)

"Generally speaking" a vacuum secondary works much better with an automatic. If you go with the 750 and it's a little rich, it will need to be jetted down.
Silver 69 has a good point though. If the 650 is set up correctly, it could perform just as well. It is rare that an out-of-the-box carb will perform to it's potential. They all need fine tuning to match each particular engine combination.


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