So exactly under what circumstance is this"backfire" happening.
WOT, idle, 30mph/high gear,accelerating,closed throttle decelerating, tip-in,roll on in first, when you slam the pedal down, etc.
So remind me; when was the last time it ran great. And why don't you start there?
Was it just before you changed the bushing and regapped the plugs? Italmost obviously wasn't the filter......
