There is a good chance that you are advanced too much to perform cold. The centrifugal advance advances your timing as rpm increases. The vacuum chamber allows it to advance under light load load conditions and retards it when the vacuum drops. It may not be dong this. One way or another, a backfiring engine when it is cold is usually an indication of too much advance. The only way to really check a coil is to try a different one. But I really don't suspect the coil.
