Exhaust problem...I think?

[bellafiat]

Help... My original exhaust system was rusty and blowing so i got a new system installed..The guy at the exhaust store told me the Catalytic converter was not needed so we did not replace it..Ever since the car is running rough.
It backfires a little and hesitates almost like im running out of gas..What should I do
Its a 1982, Pininfarina 2000, fuel injected
Any ideas?

[ORFORD2004]

I remove my catalytic converter on my 83 and the only thing that change was the sound. Much louder so i put a resonator in place. Now the sound is not too loud