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I was doing a little second/third gear pull today and my buddy who was behind me said I was pouring black smoke. I also noticed my car was popping and "backfiring" a lot upon downshifts and letting off.

All I have done is the Berk Technology single tip track pipe(muffler delete). Theres no way this would cause my car to run rich, right? Any other suggestions? I just got an oil change today, if that matters.

Thanks!