poor emmission, mot failure

Hi all
I have a 1.8 mk1 golf cab, have changed the carb to a weber dmtl 32/34 carb, it has just failed its mot on very high emissions. what can cause this and is there any things i can do to reduce emissions will changing jets to 1600 engine work ?? will timing or increasing the spark help ??? please advise has im banging my head against a brick wall thanks all
jason

I know it sounds obvious but did you check/reset the mixture when you fitted the carb? And is it jetted for your engine (did you buy is set up for a 1.8 carb Mk1 Golf)?