I've got an MC28 I'm rebuilding and when I took the pipes off I measured across the hole which butts up to the barrel. It was something like 32mm or something, I forget now. Then I measures the opening to the carb and it was something like 34mm.
Is the metal at the head of the exhaust the restriction? So can I get like a grinder attachment on my drill and grind it out a couple of mm to get more flow?
I have a lathe and I could use a boring head to ream the hole out but the pipes are such a wobbly shape I don't think I'll be able to hold it squarely in the lathe.
Here's a skanky picture of the pipe hole
I've since de coked the pipes with a heat gun. It melted the heat gun but nice coke free pipes.
Cheers
Rich