Understanding stumble at start of drag race

[SDBOB]

I've been trying to diagnose a stumble when launchING car. It's a 73 SD.The carb and engine are stock.The car has 3.73 gear, m40 with 3400 converter,9" slicks.Car starts gine idles fine shuts off fine goes through traps with 6psi.The carb has your pump,and air dashpot.I've tried to adjust air valve up and down, no change.It must be in accelerator pump.About 90% of time this happens occasion it will leave without stumble.I need direction .thanks

[Cliff Ruggles]

Did you install a new secondary plastic cam and spring?

Is the link holding the secondary flaps tightly shut at idle/high vacuum?

Are you using solid or slotted secondary flaps?

Sometimes it's not a tuning issue.  If the car leaves hard enough it may be spewing some fuel up past the rear metering rods over into the secondaries.  I put a piece of electrical tape over the gasket then poke the rods thru the tape with some grease on them after assembly.

Could also be hanger height, or the metering rods themselves.

You can also try shortening the pump shaft to increase pump shot some.

I've seen a lot of other things cause this unrelated to the carburetor. 

Drop base air cleaners/air cleaner lid too close to the carb.

Spacer or spacers under the carb.

Cutting down or modifying the divider under the carb (dual plane intakes)......Cliff

[SDBOB]

I leave off idle.When I go to press pedal there is nothing then acceleration.Yes it will pick both tires off the pavement,just slightly. It's before that. The cam is original. Carb was new has about 200 runs.I looked at cam, here goes,doesn't look deformed from wear or heat.I do have a new one from u.I do burn out slowly move to line letting bowl fill.Stage and launch.I'm installing new hi flow regulator.Thinking fuel,so I'm chasing anything.I'll check secondary closure tonight.If I remember the secondary hanger is K ,rods are machined down to 18 thou. Wish I could try another carb that works so I have a base to go by.I even thought let u you use my carb and see how it performs. I was thinking to rich,not enough steady fuel flow.I guess start at beginning. Thanks

[SDBOB]

Everything is stock. No spacers or cut down intake just single gasket. The carb is stk SD.

[SDBOB]

Slotted secondary stock flaps. I've been hesitant about modifing carb.I am now thinking buy a 76 core from you,doesn't need restored.

[Cliff Ruggles]

I use and prefer the solid flaps for drag racing.

How quickly does the pull-off release.  The pull-off release time is critical for drag racing on foot-brake cars as it helps prevent the flaps from "whipping" open too quickly and causing a momentary lean condition going to full throttle right off idle.....Cliff

[SDBOB]

Called you Tue. ,left message,about buying one of your carbs already setup.Saving this carb for restoration car. Thanks for reply waiting to order carb.