12a Bridgported, Holley carb setup
 

I run a 12a bridge-port motor in a 680# sand rail, 300' sand drags. It currently has a racing beat intake, ported with a 700 Holley Hp double pumper carb. and a plate nitros system. The car has a snowmobile clutch drive, no tranny! The clutch engages at roughly 4200 rpm's.
I have been chasing a stumble when I hit the gas that just does not want to go away. Tried 3 different carbs with the same result. I was hoping to compare notes with someone that has experience with a Holley on a BP N/A setup.

After 2 days of testing, I ended up with the following: fuel pressure is at 6.5 psi. It pulls 12-14" of vaccum at idle (idle at 2400-2600 rpm's), it now has a 10.5 power valve in it. Primary side has .060 jets and a .028 squirter, black cam with a 25cc accelerator pump, the secondary side has .076 jets and a .025 squirter, red cam with a 50cc accelerator pump. Timing at 18 degrees for start retarding to 12 degrees for launch. Hitting the nitros manually after the tach hits 6-7000 rpm's.

This combination has gotten me the best throttle response. If I turn the idle up to about 4,000 rpm's or come on the throttle more slowly there is no stumble. Running motor only, the plugs show just a tad rich. I was running the nitrous as a dry system, with a .031 pill and the gas side plugged off. My 60' and et didn't change much due to the stumble but I picked the speed up from 64 to 70 mph and gained about 2,000 rpm's.

My normal setup includes a 100 hp "tune up" on the nitros system.

Intake manifold has a large single chamber plenum?
On my old 12a bridge I only ran a 600cfm double pumper.
On my 13bpport I have a 650cfm double pumper. No power valve. 28 squirters. It likes bigger squirters. I don't really floor it off idle. I blip it up and stab it around 6000rpm.
On the dyno I started with 20 degrees timing. Made pulls without changing fuel at first. Just added timing. It would stumble at 20 degrees on throttle tip in. As I put timing in the stumble went away. Then I started pulling fuel out.

I ran a 650 on my old stock port 12a. Had PV block off, .071 jets in the primary, .076 in the secondary, and around .045 squirters on both. Idle was set at 36-3800. Clutch engaging at 4200. Had a slight flutter at launch, but came right out of it. Would hit 8900 rpm's within a fraction of a second and stay there the rest of the pass. 60' was in the 1.7 range with 76mph at the 300'. Bolted this same setup onto the BP motor and could not get it to idle. With the 700 carb, so far my 60' has been in the 2.2 range due to the stumble and 70-71 mph.

I started out with 22 degrees of timing, went up to 28 and back to 18 where i found a sweet spot.
i have an MSD adjustable timing controller on the car and have played around with advancing and retarding while going down the track. It starts best at 18, but if I advance it , it goes flat.. retarding it cleared up after the launch.
I have been starting it at 18 and retarding it to 12 degrees after warm up and then launching it from there.

Had an epiphany in the off season.
I have been trying to get bottom end throttle response out of a full bridge-ported motor that makes no power on the bottom end!
I have put a stiffer spring in the primary clutch which has allowed me to raise the idle back to around 3800 rpm's (so far) and raised the clutch engagement to 5500-5800 rpm's. First race with this change I lowered my Et by almost .5 second.
Shimming the primary to gain another 2-300 rpm's on the engagement and raise the idle to closer to 4200 rpm's for this weekend. Also adjusting the secondary clutch to pull the motor down from 10,400 rpm's to more in the 9500-9800 rpm range.
Still playing with the jet (up to .068p / .076 s) and pump shot combination, still gaining after each adjustment so far. With the 700 cfm Holley it is still showing lean on the plugs.

would love to get more info on your nitro setup. looking to put a solid setup on my downdraft semi p bridge na motor.