Valve guides have a hard time on these single overhead cam engines, quite often revved hard from cold, before the oil has reached the cylinder head. Smoking on the overrun, or after ticking over is indicative of worn valve guides. It is good policy to replace these when overhauling or re-building your 50cc or 70cc engine, and of course using the genuine Honda items
Note: this part is superseded by 12023035405.
