that 600 is just right.
seems like you need to check if your fuel pump is working. check for filter fuel filter condition if jets aren't clogged. same with your accelerator pump, check if it squirts uniformly. after that, check your mixture settings, etc.... almost all of hessitations and tendancy to stall are caused by improper fuel flow. oh, may i ask, does it smoothly pick-up RPM if you open the throttle gradually?
the best way to see if your carb is faulty is to borrow a carb of the same make and do a run-up. this way, you are sure what your carb's condition is.