Hi, guys.
I had a lot of trouble setting up my webbers as well, same thing popping and carring on. and yea they do have secondary venturies, I have replaced mine with smaller ones for higher air speed not volume which was another problem I had. anyway about the popping, if the valves are good, and you will tell straight away by a compression test ( make sure for the test you keep a wide open throttle at all times), my carbies were popping due to the twin carbs being out of ballance with each other... Very tricky to set up (especially if ya not to sure what your doing, like I was) once they were ballanced all was good, new main venturies, secondary venturies, emoultion tubes, main jets, air bleed jets, almost everything. but it was worth it. I have been complemented on how well it goes for a 2 litre and they are supprised when I tell them it is only a 1.2 litre.... ;)