The fuel mix is fine, don't change a thing there.
I am not personally familiar with that model but to check if the timing advance is set right it probably goes like this. With the motor off, turn the throttle grip and you will notice that a metal plate (throttle cam) rotates and eventually starts hitting a roller on your carburetor. This is how the motor gets more gas. Since the spark timing needs to change as the RPMs of the motor change, you need to ensure the carburetor is synchronized with the timing advance. If it is, you will notice that there should be a mark on that throttle cam (a line or an arrow) that should be in the middle of the carb's roller JUST as the roller hits the cam. If it is in the middle you are set properly. If it is not, then there will be some screw or nut that will adjust the cam so that it is.
What I am looking for here, is that when you are idled down you will notice that the cam is not even touching the carb roller. That means the carb butterfly is fully closed. So the question is, why does the RPMs continue to reduce when you turn the throttle grip slower and slower. That is because the timing of your spark is coming later and later and eventually it comes so late the motor will die if you turn the throttle grip too far towards slow.
If the timing was off too much, you might find that the late spark might provide enough power to keep the motor idling but just not enough when you pop it in gear.
Anyway, check it out. It is probably set fine and if so, adjust the rich/lean adjust a little more rich (1/2 turn to 1 full turn counter-clockwise) and see if that helps. If it does, do the carb adjustment procedure in the link I gave you and if it has no beneficial effect, set the carb back where it was and report back.
