MSD question

[outlaw08]

I have a sbc in a modified i just traded for. 421 pro action heads, full msd.my question is i can fire this thing up n it runs great. idles perfect ,revs quick and sounds perfect. BUT AFTER IT WARMS MAYBE 5 MINUTES IT PICKS UP REALLY BAD MISS sounds bout like a rag covering carb. Now i've run valves , checked compression, changed plugs ,completely rewired car , cleaned all grounds , no vaccum leaks ,
2 diff carbs. put new msd black coil and pickup in dist.my question is is it possible for 6al msd box to run good cold and not when warm , cool off and run good again . Also to hear it run it defintly seems to be electrical . any thoughts

[superstock50]

I've had the same problem.. I changed boxes and fixed it, I'm not saying that is your problem, but it fixed mine

[outlaw08]

THANKS 4 THE REPLY, I have switched from alky to gas so fuel system completely new,i've just never saw a box act this way but its about the only thing left. again thanks

[Lizardracing]

MSD will check the boxes for a fee. They run them at various rpm for 8 hours straight. If they don't find it, it ain't broke.