Mk IV 1 1/2 litre engine 'running on'

A 1 1/2 that I once owned periodically would ‘run on’ once the engine was turned off after a reasonable drive. The new owner informs me it still does
The engine has been fully reconditioned so I don’t believe it would be related to carbon buildup within the engine. The engine was tuned as best as possible and runs well (not over rich)
Any other causes? Incorrect plugs, timing?
Any thoughts.