Sorry for the silliness.

MC28 carbs can be safely bored to 33.5mm, but you have to make sure that it is bored with a vertical offset of 0.75mm.

What this means is that you don't want the bottom of the bore to be cut as the slide won't seal properly.

You can go over 33.5mm but you'll then cut into the choke orifice.

I've gone to 34mm on a set of 28 carbs for Paul, but I've had to modify the choke system to prevent airleaks.