My buddy has a ls6 in his trans am and it is missfiring, I want 2 help him out. What is the most common cause? He also says that it is running too rich, what's the best way to lean it out?

 

first thing to do is scan it to see if it's a single cylinder missing, or if its random multiple cylinders.

could be a bad sensor, burned plug wire(s), bad plugs, coil

 

I've burned the #6 plug wire several times after my ls6 longblock install...but I also installed a set of Hooker shorty headers that the plug wires almost rest on, so that's my problem. AutoTap it and it'll tell you what's going on.