Running issues/possible metering head fueling set up

hi Ive recently done a full rebuild on my mk1 golf 1.8 gti top and bottom end. I've had the metering head off and cant seem to get the fueling mixture correct its either this or timing slightly off. Is there anywhere i can get the metering head refurbed if so where and how much are my looking at or any suggestions would be great full. Ive booked the car in for a crypton tune next week hoping this will solve a few issues. Any advise would be appreciated.