Black mark on rear bumper

[dslt]

Hey guys, just wondering how can i get my car not running rich? My rear bumper has black marks and its getting kinda annoying washing it every week

cheers

[xtat1k]

what petrol do you use ?

[dslt]

BP ultimate or 100 V racing

[xtat1k]

BP ultimate is much cleaner, my car doesnt normally run rich but with V power it usually leaves black marks anyway. How long since this has been happening ?

[dslt]

ever since i put my bc spec 2 exhaust on about 3 months ago