Re: 78 triumph leaking oil left side
Not an expert and it's not gospel but:
On my 72 OIF 750 bonny I had a small tear in the primary outside cover gasket that caused a left side oil leak.
replaced oval shaped gasket problem solved.
Not sure if things where different for your year.
This was just for the final and probably goes without saying
, +1 on not plugging that internal oiler as it will blow shit up.