What plugs to use with a header on a s13

[Peabody]

Hello, 1990 s13 original engine with a header, air intake, no cat exhaust, performance chip. I have had these mods for years and have used NGK plugs. Should I use a step up to a hotter plug because I have a header on the car? The plugs come out clean but I do see black carbon on the exhaust tips so it could be running a little rich. Would changing to a hotter plug help out any? Should I get the next step hotter or 2 steps hotter. They have many models of plugs and want to know which series is better for the KA24E.

THANKS