fuel mixture problem?

I have just got a custom cat-back system and now the check engine light is on. and I also notice black stuff being shot out of the exhaust becase i can see it on the ground. and it seem to have more gasoline smell from the exhaust

When I let go of the gas pedal and still in gear and moving, i can sometimes hear faint poping noise from exhaust.

also when i tap the gas pedal quickly the engine sounds like its gonna stall.

I belive the mixture is too rich, My question is How do i adjust the fuel air mixture on the PGM-FI system? Or is it some other problem?

oh one more thing, now that i got an intake and the cat-back exhaust for my prelude, no matter how i adjust it, the Idle wont stay the same.

Its a 1991 Prelude with a B21A1 engine.