Overfuelling On Open Throttle.

[rickybiggs99 0]

The other day me and a mate tried to tune his car with a exhaust gas analyser and found it was running too rich with the mixture screw turned fully out so we thought his afm might be buggered.We took one off his other car and managed to get the co reading to bang on 1% but then when he revved the engine lots o black smoke poured out of the exhaust.Any ideas what could be causing it to overfuel on open throttle?

 

His car is a J reg 205 1.6 gti and the only mods are an induction kit and a boost valve.The bottom end is a rebuilt one from hiflow heads and he tells me his mate put a 405 1.6 head on it for him which I would have thought would have restricted air flow but I'm no expert.