402 older engine Back fires.
 

My son has an older "newly rebuilt" 402, new distributor, new power unit in the distributor. After renewing all the timing gears and associated, we finally tried to start. As it started turning good it back fired loudly, Thinking that the timing was off we re looked and found it was off quite a bit, Timing light and spark checker on and fresh gas, oils up. When it started to turn well it did a back fire again.
I and my son has both built and rebuilt engines over the years. We just can't get a handle on the non starting.
Any comments are welcome here.
Thanks for looking.