Hi All!

I have a tired old 750 double pumper that I'm looking to replace (worn shafts, warped metering blocks, etc). I'm comtemplating a 750 vac sec (its cheaper for one) but I don't wanna lose any performance. I have a 72 with a 454 (mild) and a 4spd w/3:55's. Runs great, but hard on gas even just cruising around.

Has anybody w/a double pumper went with a vac sec and been dissapointed?

thanks
Keith

Keith, I doubt that you'll be disappointed. I went the same way, and I actually picked up some performance. It's more responsive. There were a couple other minor changes, so I cant say it was all the carb, but Im sure that was the biggest part of it. If it's not strictly a drag car, I think that's the way to go. I know there are others who are happy with mech. secondaries, but I'll stick with the vacuum. John

i went from a 800 double pump to a 750 vacuum carb and loved it on the street. car was more responsive and crisp. i would invest in the quick change kit for secondaries though. mike

I've actually gone quicker with a 750 vaccum sec. vs. a 750 double pump
all most identical jetting and pv arrangement too.One full MPH faster as well with vacuum.
The perception that the car is going faster because you can open the secondaries at will (or sooner) with a double pumper is false in my experience.
The dp just "feels" faster

I'd recommend a mechanical IF you had deeper gears. 3.55 is pretty high, so I'd go with the vacuum.

I agree with those above... Once you get it tuned, there really will be no performance loss... There might be a perception of loss because you don't get the secondary pump shot, but that really doesn't improve performance...

Quick change secondary and a spring kit are a good idea...

A double pumper gives the perception of being quicker because you usually feel the secondaries open and the shot from the secondary pump. On a properly tuned vacuum secondary carb you won't feel the secondaries open, it will be a smooth transition because they only open as the car actually "needs" them to. With full exhaust you WILL hear them open though and I'll take the wail of a large CFM 4 Bbl. at WOT over loud exhaust any day.

from the description of your 750dp, i'd say a 600 vac sec would be an improvement. seriously, with the mild gears and a 4-spd a 3310 might work better. for all out performance there's no way a vac sec carb can compare to a dp though. that initial "feel" when you stomp a dp isn't an illusion if you've got the gear and convertor to match, and the carb is properly sized for the combo. i usually see from 1 tenth to 2 1/2 tenths depending on he combo, dp always outperforming the vac sec. but i've got a loose convertor and 4.10 gears.

I went a .10 faster on a 780 vac. secondary then I did on a fully prepped race 750 dp on a 396 with 4.88 gears. Do exactly what was said before and put the quick change spring kit on and tune the springs. I kept going lighter on the springs until I felt a smooth transition when the secondaries opened up. Then went to the track and did an A-B-A test was actually surprised the 780 was quicker by a tenth.