Im running my 56er ~21*ign and big carbs with 1.8mm mains. Ex ports are chamfered and comp is 160psi.
I can run up to 8000rpm allday long with this combination.

We have also 49er 75hp, chamfered ex ports, 1.7mm lowered head and big 2cyl carbs with 1.4mm mains. Ign is 20* and its running 7700rpm with 23p cleaver.