Assuming the porting was done properly, it would increase the flow of air into the cylinders, hence requiring additional fuel. One more reason why stepping up the idle jets should be your first move. Webers are very sensitive to engine mods and environmental changes, it wouldn't surprise me at all that your stumble is a direct result of the porting. BTW, I checked my setup this weekend, I have #55 idle jets in mine and the bleed screws are 3/4 turn open on all barrels. I have not done any porting on my heads or manifold, but I would expect that when I do it will require additional carb tuning afterwards.
Mike
