[Bruce Edwards]

With a lighter car the engine demand is low and as such the vac second will work better. If the demand is low and you step on the double pumps with low air speed in the intake the engine can not help but stubble even if you do not feel it. Just because it sounds fast does not make it so.
Case in point the old Quadrajets Even though it was not a double pumper and had an upper air valve to improve the air speed, like the Carter or now the Edelbrock carbs, if the valve was not set correctly and fell open when you stepped on it you got the ever famous QuadraBOG.
The way Holley and some other carbs get around the bog with out an air valve is by adding the extra shot of fuel thus the double pump carb.
Again great at high speed when the air flow is up and really sucks when air speed is low. As pointed out in a post the vac will only pull in what the engine can handle and thus you get a much smoother powerband. As the air flow goes up and the demand goes up the seconds open up so you do not get the big dump of fuel when you step on it and the resulting bog.
Doubles, as I have said before, are great for the track when you never let the rpm down and the engine needs the extra shot of fuel from the rear seconds when you pull out of the corners or for that hole shot off the line at the 1/4 mile strip when you are coming off the line at 3500 rpm or more.
For street use stick to the vacs. Less work to tune, better fuel mileage and just a hell of a lot more fun.
Again my feelings on the matter from 30 years of building engines and replacing a lot of double pump carbs on street engines with vac seconds.
Just a question to close out this post for any that may not see it this way. Look at any multiple carb factory engine and what do you see more than half the time? From 427 Ford with two Holley's, 440 or 340 sixpacks, to 427 Chevy what do you find? Vac seconds.
Ever ask yourself why that is?
By the way works even better on big heavy street cars for all the same reasons.