I just drove my 72 Challenger When I sit at a light then take off it stumbles like a fouled plug then cleans up and runs fine next light same thing at cruise speed it runs perfect. I got on it pretty hard and at the next light the stumble was gone. I cruised for a little bit and kept it mello and at the next light it stumbled. 440 source recommended Champion RJ12YC plugs I put the Autolite version of it in but it was too cold so I went up one heat range and it ran much much better. Should I go up one more heat range and see how it does? or could I have something else going on? Everything is new on the car or about 500 miles old. I am running MSD billet dist, MSD 6AL, MSD wires, MSD blaster 2 coil, Holley 950 HP carb, 512 CID with stealth heads, 11.3:1 comp, cam is Comp cams grind # XE295HL-10 and 3,500 rpm stall. Engine temps sat at about 165-170 ( it's cold here)it normally runs 175-180

It sounds like a carb or fuel system problem.
Are you using a mechanical fuel pump?
Your combination should never stumble unless your running out of gas

No I'm using a holley electric with a bypass regulator I don't think it's a fuel problem because I can spin it up to 5500 rpm without a miss or hesitation. could it just be loading up? maybe stumble was the wrong term to use like I said it feels like a fouled plug that comesback to life or 1 cyl shutting down

Sounds like you have a good ignition system?
If your spark plugs are fowling I would think the carb needs more tuning. Check the float levels with the fuel pump on and make sure the bowls are not over filling from a possable bad needle/seat, bad )-ring on the needle and seat, or possably a bad float. If changing accelerator pump settings/discharge nozzles does not fix the problem, you may have to change the idle restriction size for the transition circuit to get the correct off idle fuel mixture.
I recommend the Smothers DVD on how to super tune your car. See the Hot deals section.
https://board.moparts.org/ubbthreads/show...e=1#Post5508685

FWIW, I have had good luck with the NGK V-Power spark plugs.

Hope you meant RC12YC not RJ.

Assuming the car has been sitting (or not driven much from the recent winter), I would assume the old gas needs to be run through.

Every spring my car doesn't seem to do well, but once I get it going and get some fresh gas in....it seems to wake up.

ROAD TRIP!!

eight

Sounds like the common "off idle stumble" where your butterflies are not 'properly' adjusted and may expose too much transfer slot. If you haven't verified how much they are open at idle, that is definitely something to check.

The plugs are RC12YC sorry
Rapid I'm running a holley 950 HP I don't have a choke. This is not the off idle stumble caused by accel pump type problem. When I take off it doesn't fall over at all it just feels like a cyl is not firing then at about 3,000 rpm it comes back on and starts firing and the stumble is gone.

How do the plugs actually look when you changed them? Have you looked at one recently? Is your intake sealed good? No oil in the chamber?

Put champions in like 440 source says

I'm Pulling the plugs right now I will let you know what I find

My plugs are clean and according to this site http://4secondsflat.com/Spark_plug_reading.html they are too cold again. I barely have black burnt stuff on the very top thread of the plug not even close to the threads. I am going turn my fan temps up and take it for a drive and I will look at them again

If your truly around 11ish comp. you don't want a hot plug. I think Champs are some of the hottest.

I turned my fan temps up and it ran fine maybe The engine was just running too cold. It's a little warmer today than it was last night so i'm sure that helped alot. I'll let you know how it does tonight