serious carb issues

When choosing a power valve, make sure that it is at least 2 # lower than the vacuum reading in gear.

As for idle mixture screws, I have found that after you turn all 4 corners out the same amount to get the best idle in neutral, turn them out about a 1/2 turn more for best idle in-gear. Then you can turn down the main idle screw to try for the lowerst rpm.

As was mentioned, the secondary idle slots should be visible, but not more than about .060". That takes a while of playing with to get it right for me.

Wouldn't hurt to totally disassemble the carb and blow through ALL the passages in all the parts with a can of carb cleaner and nozzle, including the main body air bleeds. (Watch your eyes.)

What about applying as much of the tune-up as possible from the 650, like power valve, pump cams and adjustments? You could need bigger squirters and jets than the 650 because the 750 wouldn't have a vacuum signal as strong as the 650.