i have a 72cc agility and i fill her up to the brim without ill affect, no stalling or any other problems. when mine went through a stage of stalling i found out it the valves were too tight. do you have any other problems for example..running rough, sluggishness and general all round crappery? you might also want to have a look into your cvt give it a good clean out and general check up (if it needs it)
oh and before i forget...compression test the B*tch lol