You have unburnt fuel in your exhaust which ignites when it reaches fresh air behind the car:
Either your timing is out and the exhaust valve opens before the fuel has burnt,
Your valve seats are corroded and not holdingthe fuel in the chamber long enough to burn off.
Your fuel injection or carburretor is running rich and putting too much fuel in for the amount of air in the chamber
if its a turbo your ECU is not matching your airflow to your fuel mixture.
Im not a mechanic so possibly ask Reme or a proper mechanic.