My 78 cx runs well but I can only open the throtle very slowly, especially after 3000 rpm or it will sputter and realy lag badly, if I give more gas it just starts backfiring, but never cuts out completely. If I accelerate really slowly and carefully, this does not happen. However it happens in nuetral as well, albeit not as badly. anyone got any idea whats wrong? It hasnĀ“t been serviced professionaly in a long time.