if it has stock carbs, the thumb lever shouldnt be stiff at all, if both cables (clutch/throttle) are stiff and look some what old, get some new ones. new cables can make a world of difference if your ones have some time on them. has it still got the tors ? (big box things on top of the carbs with an flat blade screw diver adjuster on top, you adjust from above, removing your seat)
from closed lid, to open and k/n you will need to rejet. youll need to go up a few sizes.