Don't.Checked the plugs and looks like they are fouled a bit. N3Cs. Contemplating going up to N5C.
Pending more photos from different angles, while the engine appears to be T140V (splayed inlet ports stud-mounted Amal Mk.1 Concentric carburettors), it isn't a '79 (T140E, parallel inlet ports spigot-mounted Amal Mk.2 carburettors). If you are familiar with Japanese bikes and NGK plugs heat range numbers, Champion go in the opposite direction - 3 is a cold plug for hot engines, 5 is a hotter plug for cooler engines. 3 is correct for a T140V, a 5 will likely hole a piston. Fix the (likely) carburation problem that's fouling the plugs?
If you haven't had a Triumph before, in the internet forums a year means the manufacturing year, not the year the bike was first titled. In your bike's case, both engine and frame numbers should have two letters and five numbers, with model code (T140V?) somewhere around:-'73 Bobber
has a '79 T140V
. '73 (frame?) should probably has the ID letters and numbers in a line on the left-hand side of the steering head, aligned towards the front downtube, the second of the two letters should be "H".
. Last year of specifically the T140V was '78. Engine ID is likely on two lines on a raised area of the left-hand crankcase just under the block, second of the two letters should be "X".