Saturday I replaced the converter on my B13 with a Catco. Later on that night I was pulling out of a parking lot when my car skips a little and backfires very loud. What the hell was that it has never did that before. I changed the plugs Sunday it really hasnt done it since but what was that stumble then huge backfire?

sounds like a real nasty missfire allright which caused fuel buildup in the exhaust. Changing your plugs was spot on and if it continues then you may need to look at your wires, cap, rotor and or injectors. The new cat really should not have any ill-effects like that.

Cossie