Thread: Vacuum vs mechanical secondary

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Hello there, I'm racing a 3/4 Ton pickup with 4:10 gears, with a slightly higher stall than factory. I have a stock 454 with a performer intake and peanut port heads. Right now I using a 700 double pumper on it. It seems to be a really finicky carb. Some days it takes off hard and some days in stubles. I thought it was too rich cause it was doing it on hot days. But this past weekend it was cool and it did it a little worse. By the time it gets done stubleing the other pickup is a half a length ahead of me. I don't really want to buy another carb. I don't have much experience with carbs. But are all double pumpers this touchy. I was thinking of just getting a vacuum secondary. But I don't know if they will be any better. I need the power right out of the hole thats why I would like to stay with the double pumper if I could get it to work right all the time. Any advice is appreciated.

Originally posted by sawking
Hello there, I'm racing a 3/4 Ton pickup with 4:10 gears, with a slightly higher stall than factory. I have a stock 454 with a performer intake and peanut port heads. Right now I using a 700 double pumper on it. It seems to be a really finicky carb. Some days it takes off hard and some days in stubles. I thought it was too rich cause it was doing it on hot days. But this past weekend it was cool and it did it a little worse. By the time it gets done stubleing the other pickup is a half a length ahead of me. I don't really want to buy another carb. I don't have much experience with carbs. But are all double pumpers this touchy. I was thinking of just getting a vacuum secondary. But I don't know if they will be any better. I need the power right out of the hole thats why I would like to stay with the double pumper if I could get it to work right all the time. Any advice is appreciated.

eliminate the rear pump and see what it does.

If I do that won't air on its own make it bog.

Originally posted by sawking
If I do that won't air on its own make it bog.

the secondarys will still work, you just want have the rear squirter working.

In my opinion you have THE perfect application for a healthy vacuum secondary carb, say 870 cfm.

This carb with little tuning would allow you to bury the throttle at any time and allow your engine to respond more naturally. You could still get by with the double pumper if you learn to control your foot a lot more .............

Vacuum sec carbs are designed for heavy vehicles where some RPMs need be built before indulging the cylinders ...........

you may need more fuel to little fuel will do this also. if u punch the gas out of the hole and it bogs try putting a quicker pump cam on the primaries and maybe the sec. should help

I suggest you find a copy of the Holley carburetor handbook 4150 & 4160 selection, tuning & repair by Mike Urich HP books. It's a pretty comprehesive guide for a 4150 which is what a double pumper is.

One of the problem could be fuel slosh from the bowl vents use a piece of fuel hose between the vents with a noch in the top center. 2.since you are racing it would be a good idea to use jet extentions on the secondary jets to prevent starvation. Do you have the corect power valve . As mentioned change the pump cam position and try larger or smaller squrter sizes. Hope this helps Dave