Black smoke cloud

[ectomorph]

ok so today i was driving home and spotted a buddy of mine and decided to pass him and slow down, almost to a stop. the car next to me was going just as slow so he wouldnt be able to pass. i downshifted to second taking off(going about 35 when i downshifted) and i see a cloud of smoke in my rear view mirror but i cant tell what color it was because of my tint. the car drives normal but the smoke worries me. he txted me saying, "dude you had a crap load of black smoke come out when you downshifted." sorry for the story.

I'm just worried and want to know what might cause this. i know i have an exhaust leak but i dont think that'll lead to that much smoke. i'm no wiz at fixing cars but i'm learning. i want to know what might cause it and how to fix the problem.

oh and i've had my 240 for about 3 months now.

[nifares240]

that means you're throwing unburnt gas.

[ectomorph]

so it is my exhuast leak?

[Wc240]

where is the exhoust leak? before the o2 sensor?