Have an 81 Cutlass with 267 Chev engine. When its cold (most of the time here in Winnipeg, Canada) it cranks and cranks without firing. Have changed plugs, cap, rotor, fuel filter. It will start if block heater is plugged in. Can anyone explain why it will fire when plugged in and wont fire when its not? Any suggestions gratefully appreciated.

You need to change the timing by rotating the distributor.

By do so you will lose a little performance, but she will start for you