oil coming out carb

i have an arctic cat 500 manual a 2000yr model, oil coming out ofcarb while running ,it happened to me 1 time took it to shop and said carb needed cleaning(they cleaned it and has new gas). lasted two weeks and a total of running it three times for maybe 20 min each, back to same problem. i took carb off cleaned it best i could put it back on still doing it. the motor has 600 hrs on it. any suggestions?? please help

Check the vent tubes coming out from the motor and see if you have a vac problem.