I have a customer who does some of their own work. They have a Mitsu. FGC30N / AF83F10420, they recently replaced the cam due to it breaking. Not sure what caused it to break. They installed a "good used one" and had someone else put the timing chain on and set the timing. They have changed all the spark plugs, coils, cam & crank sensors, cleaned the primary injector, made sure Oring was not damaged, checked fuel pressure (4.0 PSI), checked wiring to assist injector. When you ease into the throttle it will accelerate perfectly, but if you slam the accelerator it will pop (backfire) through the air filter box. I was wondering if it could be the fact, they did not change the lifters or if there is an issue with the assist injector failing as they will not code out.