240 having problems need help ASAP

[Florida240sx]

He was on the road yesterday and was going WOT probably had a buddy behind/ with him.When he hit 6000rpm in 3rd his car sputtered then blew out a puff a big black smoke. It runs fine at idle.But I guess if he gives it gas it will sputter and make mor eblack smoke.He's thinking bad gas.What should I do?Going to pull the plugs when he gets here which is in like 5 minutes. Thanx for your help in advance.

[Florida240sx]

Only does it at WOT under load at 6000rpm.Could this possibly be the fuel filter?Goign to post this in nissan online mechanic sorry but help here would be helpful too.