Welcome to the Forum.
Mine is XX02646. I bought it as a T140V- did you mean 1/1/79, or is it a 140E?
'78 and '79 T140's can be confusing, not helped by we (and Triumph) refer to the 'Model Year' (aka 'season') not the calendar year.
The "XX" indicates a 1978 Model Year (the right-most "X") bike built in December (the left-most "X"). However, because Meriden usually changed from building one Model Year's bikes to the next Model Year's bikes during the summer, your bike was actually built in calendar
Complicating this particularly in the 1978 Model Year was the US Federal government introduced new FMVSS regs. on 1st January 1978 applying to all vehicles built
after that date. These regs. particularly stiffened the emissions requirements and changed the lighting requirements (always-on); as "Kadutz posted, to meet them, Triumph started producing a version of
the T140E just for the US. It's this version of the T140E that causes most consternation and likely isn't even mentioned in the Haynes manual.
Returning to your bike, as it was built
before 1st January 1978 is probably why it could be imported and sold in the US during calendar 1978. If it's 'correct', it has a 'splayed port' cylinder head - the carbs. stick out at an angle from the centreline of the bike - you press a tickler button on each 'til fuel squirts out before trying to start the engine from cold; the main engine breather is a large-diameter pipe clipped around the edge of the rear fender, one end attached to the primary chaincase under the left-hand carb., the other end open to atmosphere behind the rear license plate?
rewiring my T 140 V
which wiring diagram
two in my Haynes Manual, for one switch or for two switch models.
( I don't even know which switches this refers to!)
I'll say "two switch" - 2-position (off/on) key-operated ignition switch in the left-hand headlamp mounting and 3-position (off/tail-'n'-pilot/head, originally at least) toggle-operated lighting switch in the top of the headlamp shell.
Digressing, the "one switch" likely refers to the '79-on second version of the T140E, that has a 4-position key-operated combined ignition and lighting switch mounted with the idiot lamps in a console between the speedo. 'n' tacho.
If your bike's electrics are "two switch", it also has:-
. A single Brown/Blue (insulation mainly Brown with one or two thin Blue stripes) wire attached specifically to the battery negative
terminal; one or more specifically Red-insulated wire(s) attached to battery positive?
. Red wires in many places on the bike attached either to or near electrical components?
. The alternator cable - emerges from the primary chaincase under the left-hand carb. - attaches to the rectifier with a White/Green wire and a Green/Yellow wire?
. Inside the headlamp, the Lighting toggle switch has a White and a Blue wire attached to adjacent spade terminals at one end of the switch, and a Brown/White and Brown/Green wires attached to adjacent spade terminals towards the other end?
If you can check the above, click here
for a free download of the Triumph '73-'78 750 twin workshop manual, the correct wiring diagram is on manual page H22 (.pdf page 191).
That diagram shows the Lighting toggle switch wiring connections exactly as I've described them above, attached to a switch numbered "34419". Otoh, as Kadutz mentioned, '78 T140E's have a different switch and wiring - looks similar but has a different number (31788 or 35710) and either the White or Brown/White wires won't be present. This wasn't fitted as standard to T140V's but it's an easy conversion if the first owner wanted always-on lights.
If you find that or any other differences on your bike, come back with details and we'll get you through the minefield.
rewiring my T 140 V
Standard wiring was very-much 'built to a (very low) price';
there are various inexpensive 'sensible mods.' worth making, post if you want details.
think the Haynes might be best for keeping the garage door open on a hot day!