I was doing some reading and a lot of books recommend not using a double pumper with an automatic transmission. I talked to my uncle who is a long time drag racer and he also said I should be running a vacuum secondary carburetor with my 727. He said a double pumper will be difficult to launch. Currently I have a 750 speed demon on the old 340 and the car hooks up and launches like a bear cat. I don't want to fix what's not broken in my opinion. Does anyone else run double pumpers with auto transmissions and have good success or am I just lucky?
Double pumper with automatic
- Thread starter 1969GTS
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demon seed
The Original Demon Seed
It all depends on your combination. I used to run a 750 vacuum on a 383 with a single plane, 727, a high stall converter and 3.91 gears. I had a huge off idle stumble when you mashed it to the floor, even with a 4 hole spacer. I finally decided it was telling me "more gas stupid" after playing with timing and other things so I swapped the vacuum unit for a 750 DP'er and guess what, problem went away. Now you can come off idle and fry the tires off her with narry a stumble all the way to 6500 RPM and over 100.
Basically if it's working out for ya leave it alone but for minor tweaks.
Terry
Basically if it's working out for ya leave it alone but for minor tweaks.
Terry
grumpuscreature
Resident Curmudgeon
If it ain't broke, don't fix it. "They" aren't always right ya know.
cudaman51
Well-Known Member
I have one on a near stock 340 and i cant get the trans paced wright with the carb throttle position (sec come in to soon).I am shure i will have more low end torqe and better throttle response with a vac. sec.Man valve body wouldnt be so bad.
Dp carbs are harder to launch hard on the street because you have to control the amount of open throttle. To much can be bad to real bad which of course translates into slow or real slow.
You can tune one to work and learn how to feather it, but it's a pain. The vacuum carb does it for you and does it well all the time.
The more radical the combo, the easier it becomes since grand-ma street driving is out the window and no longer a concern.
You can tune one to work and learn how to feather it, but it's a pain. The vacuum carb does it for you and does it well all the time.
The more radical the combo, the easier it becomes since grand-ma street driving is out the window and no longer a concern.
OldVart
Well-Known Member
If that DP'er is working for you, leave it alone. I've run both vac and DP with both auto and stick cars, and every combo is different. Generally speaking, the DP carbs work best with a manual tranny, but when one works good with an auto, don't mess with it. You could go from being a happy camper with your current combo to a disgruntled, cussin', driver trying to tune in a "better" carb.
BobsDart
Well-Known Member
I have a DP on a mild 360/auto. 2500 conv. 50cc pump on rear.
Car launches like a violated ape!!!! Previously had a 750vac, used my gas cruising, and a softer launch.. Like said before "ain't broke don't fix"...
bob
Car launches like a violated ape!!!! Previously had a 750vac, used my gas cruising, and a softer launch.. Like said before "ain't broke don't fix"...
bob
The fact that a carb is a double pumper is really never the problem...it is the SIZE of it that does. If the carb is too big for a combination, the performance of the car will not be optimal. I've seen a lot of times where you spend more time farting with the spring in the vacuum diaphram to get it to come in where you want it than you ever would with the double pumper. Gas mileage is not a concern because the second pump isn't involved until you get over 30-degrees of throttle opening. Heavier automatic cars tend to like the DP because the extra pump shot helps the moving. I run a fully modified 750 DP on my 340 Demon with a big solid lifter cam, ported X heads, etc. and it just loves it. It has a 727 full manual reverse pattern valve body, 4,000 stall converter and a fabricated 8-3/4 rear with 3.91 Power Lok. Lower gears really help bigger carbs get up into the power band. Like the man said: "If it ain't broke, don't fix it!".
Beatnik
Well-Known Member
Leave it! I had an old 650 DP on my 340/904 combo before swapping heads/intake last year. I then "upgraded" to a new Edelbrock 650 AVS and have since also tried a new 750 AFB...neither of which can hold a candle to the old Holley. I'd trade both Edelbrock carbs back for the DP!
The Eddy's are nice tame street carbs for relatively stock motors, but that DP would launch like crazy compared to both Edelbrocks with 292 duration cammed 340 with ported heads...
The Eddy's are nice tame street carbs for relatively stock motors, but that DP would launch like crazy compared to both Edelbrocks with 292 duration cammed 340 with ported heads...
jiw700
Scamper340
i have a 700 holley DP mechanical 2'nds, the thing ROCKS(340-904 auto)
LOL, I love it, "Size does matter!" PFogel, you are 100% correct. The combo must work!
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