440 wont start

ok heres what i have going on i changed my heads from 906's to eddys went from the 88cc 906 to a 84 cc eddy the car ran great with the 906's now i can't get it to start with the eddys on there all i get is backfiring and when i play with the timing i get backfiring thought the carb the fireing order is right i'm getting good spark but it just wont start i'm running a .30 over six pack bottom end about 10.8:1 comp i also watched the valves open and close every thing seems to be there but no fire this is my first 440 an i missing some thing any help would be a big help thanks