I have an 88 GTU, with a rebuilt engine from about 3 years ago. I am having really bad afterburn at idle, but only when it's cold out. When it's warm, and the engine is warm, the car idles fine. When it's warm, and the engine is cold, it idles fine. When it's cold, and the engine's cold, it afterburns like mad. When it's cold, and the engine's warm, it afterburns like mad. The engine has stage 2 porting, with a cold air intake, and an aftermarket exhaust (full). Anyone have any ideas/similar problem?

 

Whatever the cause you're running too rich (too much fuel/air). The engine intentionally runs richer when cold in order to hold a smoother idle, which probably adds enough fuel to the already rich mixture to afterburn. The aftermarket exhaust would only lean out (decrease) the fuel. I wouldn't know much about porting, except that it can make a rougher idle due to decreased low rpm horsepower.

Hopefully someone here will help you, but I bet they'll ask for more details.