No vacuum hose on a Thunderbird as far as I remember ...... I would guess at the fuel tap filter being blocked / restricted
This reply may well be total bollox, as I know not how this tap is set up, so please feel free to disregard it at will! I have seen, in the past, for I am old, a set up where the fuel tap is fed by by two tubes - one very short one to supply the reserve fuel, and a slightly longer one to supply the main tank, as it were, i.e. when the fuel level gets too low for the longer pipe to pick up, then it is time for the reserve to spring into action. If that is the case then I would suggest that the main pipe has a restricted flow which is by passed by using the reserve position on the tap. As I say, I have no knowledge of the workings of this particular tap but hope that the above may be of some use. Just thinking out loud really. Right ..... I'm off back under me favourite rock. Warm, safe and cosy there!!
I think all the T300 model Triumph's used vacuum fuel Taps. So the problem maybe a perished vacuum hose or the running side of the tape has a blockage. Only way to find out is remove tank , drain it , pull tap out and test tap ! Apply vacuum and see if you can get air to flow through the pipe ! There is a diaphragm valve that can perish too !
Tiger, Thunderbird, Thunderbird Sport, Adventurer and TT are all gravity feed taps with an Off position. The vacuum taps fitted to other models do not have an off position but have a Prime position instead. The fuel tap should have 2 different length tubes on top to provide the normal / reserve functions of the tap ( as @stevethegoolie mentioned ) and will have either separate filters on each tube or one segregated filter that fits over both tubes to provide filtering in the tank. You will need to drain the tank, remove the tap and check the filters to make sure they are both clean then blow through the tap or use a pipecleaner or similar ( do people use pipecleaners still ??? ) to check through the passages in the tap.
Fuel tap for the thunderbird part T2400150 fitted to the following :- Triumph, ADVENTURER (from VIN 71699), Fuel Tank and Fittings Triumph, ADVENTURER (to VIN 71698), Fuel Tank and Fittings Triumph, BONNEVILLE & T100 (carb models), Fuel Tank & Fittings-Bonneville 100 317247 Triumph, BONNEVILLE & T100 (carb models), Fuel Tank & Fittings-Bonneville 317246 Triumph, BONNEVILLE & T100 (carb models), Fuel Tank & Fittings-Bonneville 317247 Triumph, BONNEVILLE & T100 (carb models), Fuel Tank & Fittings Bonneville T100 317246 Triumph, LEGEND TT, Fuel Tank and Fittings Triumph, SCRAMBLER (carb models), Fuel Tank & Fittings 317246 Triumph, SCRAMBLER (carb models), Fuel Tank & Fittings - 317247 Triumph, THRUXTON 900 (carb models), Fuel Tank & Fittings 317247 Triumph, THRUXTON 900 (carb models), Fuel Tank & Fittings317246 Triumph, THUNDERBIRD, Fuel Tank and Fittings Triumph, THUNDERBIRD SPORT, Fuel Tank and Fittings Parts listed here :- https://ie.fowlersparts.co.uk/parts/5877/thunderbird/fuel-tank-and-fittings
