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 hasn´t been serviced professionaly in a long time.