1. Insufficient valve cleaning or poor grinding of the decompression screw 2. Gas leakage through the valve 3. Gas breakthrough through the cylinder head gasket 4. Wear, sticking or connection of piston rings
1. Lite the valve 2. Replace with a new one or wash 3. Replace with a new gasket and tighten the nuts in accordance with the rules 4. Replace damaged ones, clean stuck rings.