Without being funny, just ring/e-mail Weber... they will tell you, after all they spent ages developing these carbs and experimenting with the progression holes to ensure the best drive they could get with the Zetec engine.

Its all to do with part throttle and the transition from idles to mains.

It just means it will drive lovely and smooth with no flat spots.


Drilling your own progression holes is a dodgy game, if you drill them in the wrong place you will introduce flat spots and without a very steady hand you risk damaging the body of the carb.