Most problems can be diagnosed with the following steps:
1. compression test
2. leak test
3. Remove carb
4. check reeds
5. take a look at piston through ports while carb and reeds are off
6. clean carb x2, properly set floats upon assembly
7. check the air filter
Then put it all back together, use a new gasket on the exhaust and reed valve. Read up on leak test, it is very important for the life of your 2 stroke and should be done twice a year