Hi Allan!!
You say 12mm carb is to large??
How many have not run MAC or CMB with 12mm carb?
Why not test it as is and see how it works?
Just my thought.
Yes, i run my CMB 90 also with 12mm and that works good.
I also run 12mm on my tether car 5cc Engines so what is big??
On a 1,5cc tether...