Back fireing through the exhaust

[chevy21057]

MY grandfather has a 91 ranger that has been setting for a while. We got it out the other day and it would hardly pull itself or idle. I changed plugs, wire, fuel filter and cleaned the throttle body. Now it will idle fine but when you drive it, it will back fire through the exhaust. What controls the timing on these? Or could it be something else?

 

[whatelseisleft]

try an o2 sensor...IIRC timing is comp. controlled, does it have a dizzy?