904 Shift Problems

Sounds like the kickdown lever may have gotten stuck to far back during the burnout if you didn't have a return spring on. That would explain the shifting prob. after burnout contest. Do you have a 4bbl or stock 2bbl. Is the linkage stock. This all affects kickdown motion & proper pressure. Hopefully it is just kickdown adjustment that you need. Remove carb & kickdown springs. Set it up initially by moving kickdown rod attached to carb all the way back towards firewall, some have holes in mounting plate to insert small rod to keep it place while you adjust.(this will be max pressure) then move carb attaching point to wide open throttle & adjust linkage so it just fits on mounting point. Re-attach springs. For more pressure lengthen the rod. For less shorten it. Be carefull lengthening it to much or your carb will not open all the way. It's a tial & error thing especially with a modified setup but you ned that trans pressure right or good by trans. quick. Heres a link with some kickdown diagrams.Yours should be the last one.
http://www.moparts.org/Tech/Archive/auto/35.html
Good luck

Oldschoolcuda