You most probably have one of the following problems.
1) an air leak in your cases - take it apart and re seal everything.
2) an air leak in you carb - Check carb seating and O-ring and needle valve seals.
3) leak through you bearings - Replace bearings
4) your cylinder liner/ piston is shagged - replace (or just buy a new engine)

Do everything in that order with the cheapest being first and it might not cost too much