Ka-pow!

I have a strange issue with my rebuilt 440, on initial cold start I get a back fire out the exhaust, just one pow then it runs good,
After it warms up, I can turn it off and restart umpteen times with no backfire, heck it`ll start just using the key, no pumping accelerator.
Anybody experience this before? if so, what`s the culprit?
Timing is @ 18 initial.
Thanks, Grant