Car back fires on launch on foot brake then straightens out I changed plugs ,rotor button.
461 BBC

Well you tired most of ignition system, how about fuel delivery system? Only takes 3 thing to make engine run.air most always there,thrpm in some fuel and fire it off. You did not say you changed wires.How about advance?If you are confident ignition is in good order ,only leaves one area. FUEL

Are you getting too much fuel to motor when going WOT?Are you getting too little?
Is fuel running away from jets in bowl at hit? Pump and regulator keep up with demand?
How well was it running before issue.
When did it start?

Lots of question for carb,figure you can go over them one by one. Just trowing cpl thing out for you to look at.

Like Chris said, need more info about what the car is doing at launch. Just bogging, dying completely out where you have to restart etc.etc.
I had a 850 on a 496 that was back firing on launch, sometimes would recover and go, sometimes would die out and quit where I had to restart. Checked all electrical as you have then went to carb. Ended up going from 28/31 squirters to 37s
and going to a high lift long duration pump cam.
Hope this helps
good luck
Scott Horsley

Thanks for the advice I changed coil and wires yesterday I have cap on it's way I run E85 for fuel. The problem started about 2 weeks ago it acted up one time then was fine after that but this Saturday it started off that way. Car idles good and when you throttle up sitting still seems to be fine. Going to test and tune Thursday night since made changes then go from there

In general that means it needs more pump shot, and/or slightly richer idle. Make sure nothing is stopped up.

Did you check your ignition timing. Sounds as if it could be retarded.

When you say "backfire", is it through the exhaust, or the carb?
Is it right at the hit of the throttle, or a split second later?
What is your timing set at?
If it is more of a flat-spot or bog, probably not enough pump shot as others said, but could be due to shooter size, pump cam, or even adjustment of the pump arm. (If you are staging/leaving at say, 3,000 RPM, you could have used up most of the pump shot already, if the actuator arm is set too tight).

Check booster pump arms or levers found one missing once. Stumbled a little going up against convertor. The roll pin had worked out.

whats the water temp at when staging? needs to be above 170

Very possibly a lean backfire caused by clogged booster nozzle (sticking check valve) or too small booster.

Also you may have wrong cams for this booster squirters. They have to work together. Larger nozzles will drain quicker and my not give you the fuel when you need it. Same with too small, still squirting fuel long after the hit. Then you are just making it too fat.

Thanks for all the advise so I checked timing, then changed carbs and on about 3rd pass my buddy seen sparks at rear of car on launch so we jacked up car and found battery cable cut and smashed at section in frame then we fixed it made another pass it was smooth so put original carb back on made another run and it is working fine. Again thanks for all advise.

Good Deal.
Man, it really helps to have a buddy/team mate/crew member who can eyeball things while you make a pass. That kind of problem could be a hard one to figure out without the help.
I am always a solo operation, so I'd have probably been chasing that for a good long while.
Glad you got it fixed, and thanks for sharing the info. It may help someone else down the road some day.

Couldn't be battery cable has to be converter

Water temperature too low. I had it happen Saturday. Way to long between rounds. Cold day, should have warmed up just like start of day.