The bike has 4800 original miles on it and has been sitting in storage since 1983.
On this "first 60 miles", you revved it how high?
As a general rule, the twins' oil pumps don't allow wet-sumping; by 1979, the Co-op didn't generally make bikes that developed problems inside five thousand miles and what problem could've developed sitting in storage for thirty-six years (you have fitted new tyres and tubes)?
Plugs are NGK B8ES.
Wrong ones for a '79 T140E - they're equivalent to Champion N3, which is what were fitted originally to earlier T140V
's and other twins.
Your bike was originally fitted with Champion N5
's, NGK equivalent is B6E-something.
Note Champion and NGK heat ranges go in opposite directions - Champion, the lower the number the 'colder' the plug; NGK, the higher
the number the 'colder' the plug.
I would strongly advise fitting better than NGK S-suffixed plugs - NGK B6
EV or B6EVX - they have thin unobtanium/costalotium centre electrodes that cope better with the 'fuel' we're all flogged today and the fairly rudimentary carburation on these old heaps.
And go out and give the engine some revs. ... they aren't supposed to be chugged like Hardlys. The guy that's famous for owning Big D Triumph - Jack Wilson - advised Triumph owners to "ride it like you stole it" ...