1985 Chevy Truck truck miss fireing and back fireing

Engine problem
1985 Chevy Truck V8 Wheel Drive Type unknown Automatic 155568 miles

I would like to know what could cause this when I start my truck it runs fine but when it worms up tp about 100 it starts to jerk then back fires then when it completly warms up it stops thats at 250 and its just when it is really cold I already fixed the vacum leak.