Barend, there should be no tuning or after fitment adjustment to the carb after fitting a snorkel. The only thing that could be influencing the airflow (at idle speeds) is if there is a 'blockage' like a rag of foreign object in the snorkel. What Everdt suggests (taking off the Donaldson's cover) allows free airflow which is not through the snorkel. If this makes a difference, then there is a blockage in the snorkel.
However, my money is on the idle jet solenoid as Pieter suggested. It's more than possible that a wire to the carb was dislodged while fitting the snorkel. There are two wires to the carb .... one for the auto-choke and the other for the idle jet solenoid. Check that both are properly and securely connected.
