EXHAUST PROBLEM?

Hey guys, i have an '86 xj i just finished putting on a brand new exhaust system (everything except headers) i have a one in two out flowmaster running twin pipes out the back, a brand new catalytic converter, and just replaced the flange gaskets up front (the donut ones up front after the exhaust manifolds or before the downpipe) and im getting this really bad backfiring, its like a thumpthumpthumpthumppop coming out of the very back when i downshift and let out the clutch any idea why this would be happening?

thanks,

Jarrod

with most aftermarket exhaust you will hear more after firing(back firing). Some after firing is normal on down shifts, with engine brake cause thats when it runs rich. depending on the set up of the aftermarket exhaust, some run just a muffler and no resonator, so it will be louder.