'75 Triumph Trident T160 - Engine Still Smoking!!!

I rebuilt the engine of this Triumph Trident T160 recently and although it ran well, the engine was smoking badly. The general consensus was that the piston rings hadn't bedded in properly (as is sometimes the case with Tridents especially) and they would require replacing and possibly the barrels re-honed. However, having done all that and reassembled the engine (again) the damn thing is still smoking! It would appear that the piston rings/bores are not the problem but something to do with the cylinder head. I am therefore just beginning to strip it all down for the third time to see if I can eventually find the source of the problem.