In my '85 Westy, I opened the black box on the firewall in the engine
compartment ... the one where the charging wires from the alternator run
to a standoff ... and found that where the two wires from the alternator
are crimped to a single ring connector there was corrosion and the wires
showed evidence of overheating.

The charging wires on 1.9s are too small (bigger wires were used in
later models) so I added wire straight from the alternator to the
starter terminal.

I sometimes use a number of electrical consumers and I have installed an
aux battery, all of which cause the alternator (and its wires) to work
harder.

It wouldn't hurt for you with 1.9s to check inside your black boxes for
a similar problem, especially if you've added a battery.

Mikel
'84 GL 7 pass
'85 GL Westy