Suggest to diagnose at workshop, usually the keypoints of diagnose are:
1. Check the seal between valve and valve seat, proceed with grinding if necessary or replace with a new unit.
2. Check the worn level of the piston ring, adjust or replace piston ring if necessary.
3. Measure the worn level of cylinder liner, replace if necessary.
4. Check if the valve spring is broken, or the rebounce force has dropped significantly.
5. Check the valve gasket if there is leakgage, replace if necessary.
6. Check if the spark plug gasket seal is aged, replace if necessary.