according to the repair manual "intake backfire" is caused by improper ignition timing, or a defective EGR component, or defective TAC vacum motor or valve.? i have a 77 w/an L28 and all my starting problems have been due to a bad pick-up coil, and a loose fuse link. my symptoms for the pick up coil have been. shuting down when norm temp is reached2 not starting at all.or just running for a few minutes then just shuting down. and also cranking with no results. and for the fuse it has been gagues not working when you turn the key.