egr or timing backfire

Have you drastically increased the setting of the speed screw?
This usually points to retarded timing and often that is because the cam has gone retarded.
The quick test is to just pull in some timing and if it stops doing that, then put the speed screw back. Now you can check the ignition-timing and if it is seriously advanced then you go check the cam-timing.

But if it's only blasting fire out of one side of a dual plane, then you know where to look, namely one cylinder on that half of the intake, has a problem. Isolation, as mentioned is by removal of one hi-tension secondary wire atta time.