immisions failure

Hello all
In need of help, i have a 1.8 H reg golf cabrio last year i had a webber carb fitted, i have just had it in for MOT and it has failed misserably on the imissions being a mile out, they have tried adjusting the carb but to no joy.  can anyone give advice, know of a cheap second hand standard carb that will fit so i can try that …. i dont know what else to do    
  thankyou jason

Hi just to start has the rubber at the base
Of your carb split ? Easy way to test run
Engine spray brake cleaner at base of
carb engine will sound different or wil
cut out hav a go easy test cheers

Probably needs jetting for your particular engine?