Silly question but... change timing after swtiching ECU?

Popping in the exhaust on deceleration is possibly due to air being drawn into the headers at a faulty gasket up by the head. What happens is this; if the mixture goes rich on closed throttle, and/or the engine misfires then unburned gas gets into the headers. If it finds oxygen there, and a point of ignition, then it will ignite right there in the header, and pop when it goes out the tailpipe.. This type of popping is usually called an after-fire.
Popping in the exhaust on acceleration has a different source.
A backfire is when a flame comes out the carb. This has a third origin.