my ka problems!!!help me....

[nismo_power]

okay i have tuned it up and it seems to be still back firing...it has a gutted cat and 2.5 in cat back, with a intake...when ever i down shift to slow down it pops and when ever i shift into second it pops...i dont think its suppose to do this so can some one tell me what is wrong and how i could fix it...it sometimes lag too around 4k-4.5k...this is starting to get anoying so im going to fix it....thanks in advance...

[TDS]

backfire is common with aftermarket exhausts since the exhaust system becomes less restrictive. I actually like the backfire. You can check for leaks to reduce the backfire, if you have any leaks, but I don't know about getting rid of the backfire altogether without putting back on a stock exhaust.