classic motorcycle forum

Firstly hello to everyone I am new to the forum,although I am a skilled automotive technician I am a relative newcomer to the world of British motorcycles.I would therefore like to ask for people's opions on a fault I have on my bike.Its not strictly a triumph but is a triton with a triumph t6 bottom end and t140 barrells and head it is a pre unit from 1950s I believe.I did not build the bike but purchased it from the owner who had owned it since 67 he told me that the engine was rebuilt ten years ago using nos barrells and pistons and head and rods etc, it has since done about 2k the issue I have is smoking from the left bank (blue so its oil) worse on tick over and it fouls the plug when I look in the bore and into the intake the piston crown has a build up of sticky residue and there is engine oil in the intake track.I am thinking its a leaking or loose valve guide or a piston oil control ring fault before I pull the heads and barrells I just wanted to make sure I am not missing something obvious also the right bank is fine thank you for you're help .