I just looked out the metering orifice sizes for that 650BC carb. To me it looks like the idle air bleeds are generous and the idle feed restriction is a bit stingy , on that carb. But I am not ready to tell you to start drilling etc yet.
(I am not that familiar with that specific carb model)
You can manipulate the idle air bleeds by hand while it is running by blocking /restricting flow with your fingers.
and read the AFR gauge @2500 and 3000 rpm. Then try the same with the high speed bleed, using a mirror too.
to see the main start up rpm.
Again verify no leaks (intake,blower and exhaust gaskets) and correct idle throttle position T slot exposure @idle FIRST
and correct base idle mix adjustment (not based on the AFR reading @ idle.)