I am a retired motor mechanic whose hobby now is motor cycles .I thought I knew everything about engines but this problem has me beat. I have a 1971 Aermacchi Sport which runs beautifully but when left overnight to cool,net morning the exhaust smoke is unbelievable.When it clears, it is OK until stopped and left to cool.I have partly stripped the engine,the piston ring gaps are perfect.I have fitted new valve guides and checked for porous castings.Still the fault persists.Has anyone any suggestions ?