more big block duster tuning

Posting yet again about the same car ('75 plymouth duster w/ 400 big
block, 727 auto, 8.75" rearend, Holley 4160 600cfm vacuum secondary carb,
mild cam of unknown specs). Just about have the engine sorted out now,
with the exception of a hesitation when I punch the gas. The car does not
hesitate under light throttle application and also does not hesitate if I
give it a little gas and then stomp on it. It only happens when I give it
moderate to heavy throttle from idle. I want to suspect the carb, but I am
still learning and don't have a whole lot of tuning experience yet, so any
suggestions would be helpful. Could a rich/lean condition of the
accelerator pump be causing this condition? Thanks for the advice!

 

sackattack84 wrote:

You don't say, but I suspect you've got 3.55 or 3.92 rear gear and std
torque converter. Might be that the secondaries are opening too early.
See pg 12 of the Holley 4160 ref. here for more info:
http://www.holley.com/data/Products/Technical/199R7948-5.pdf

Also, Holley has a tech support line and on-line help, as noted below:

"Online Tech Help, If you have questions about our products, need a
part recommendation, or just cant seem to find what you need you can
contact our Tech Support online and receive answers by e-mail.

Holley Performance Products provides award-winning technical support
via phone Monday through Friday, 8AM to 5PM CST @ 270 781-9741."

See their web site for more

 

With the engine off, look down the primaries on the carb while briskly
opening the throttle and observe the pump shot.
A weak pump shot can cause the hesitation you are experiencing.

 

How does it idle?? How much vacuum at Idle? If you have a big cam I would
bet the throdle blades are too far open causing the hesitate. Sometimes
requires very small holes to be drilled in the throdle blades to let the
enging get more air without the throdle blades open past the transisiton
point. Had to do that to my 750 holly on a 360hp/360 motor.Turn the carb
upside down & look into the pri side their should be a slot in the body,
the throdle blade should be in the middle of it. A good distributor is
also a must. Just as a starting point run 10-12 degrees of inital
advance, 22 in the dist depending on compression & type of gas something
around 36 deg total with vacuum disconnected & plugged. You may find that
600 a little small for a HP 400. I used the 750 Holly & sometime the 850
thermoquad