Goog Morning Antemeridio,
if you would like to awake your Motorbike after 15 Year sleeping, you should do these:
1. emty the gas tank completely and wash it out with fresh gasolin. carefully and perfect clean. That shall eleminate all old rest components of the prehistoric old fuel.

2.   Engine: Take out the carburettor Battery for cleaning.
3.   Engine: Change Engine Oil. Cold Oil is not running very good, so Open the Drain Plug and let run out as much comes out.
3.1 Engine: Change Oil-Filter.
3.2 Engine: Take out the Spark Plugs and inspect them for condition. The better Idea, get new ones.
3.3 Engine: Open the three Screws of the cylindric Cover on left Engine side (Generator Cover).
    Open the centric Cover Screw, carefully, to get access to the Crank shaft. Check Gearbox for Neutral Position.
    Take Tools to turn the Crank Shaft clockwise direction. This shall test, whether the Crank Shaft, Pistons are free runing. It is just a Test.
    If the Crank Shift will not move, there you have a little problem. Perhaps the Piston-Ring are corroded to the Clinder inner Side and hold
    the piston(s) in its, in there position (s).

4.  Chasis: Change Brake Fluid completely.
5.  Electric: Check Battery for Condition, recharge it, or take a new one in.
6.  Re-Animation:
    a) fill in new Benzin
    b) Set Fuel Cocks to PRI-Position and flood the Carburettor Fuel Chamber. Round a Bout 1 Minute.
    c) Try to start the engine. If it will not start immediately, try it again, 1 minute later
    d) If the Engine fires, let it run. Check for leaks around the engine. Bring the engine to a good temperature and check for rising temps
        on both cylinders.
    e) if neccessary, synchonise theCarburettors
7.  If all these was successful, change the oil again, to get out the eventuely rest of the old Engine oil. In the End to get a clean inner Engine.


Basic Information you can find here for Carburettor:
https://www.tr1.de/pages/technics_manual.php?key=XV750se&lng=eng&flg=0&pge=0400  (based on XV750) for TR1 also.

If your old fuel smells like Old socks, you should insppect the fuel cocks, 2 of them. Function, clean Filtern. These Filtern are sitting on the Fuel cock and showing into the Fuel Tank. Perhaps the Filtermesh is contaminated with greasy old Fuel fragments?

2. Take out the carburettor-battery
Alternatively, you can bring the complete Carburettor to your Yamaha dealer and let clean it there. Mostly with Ultra-Sonic-Method.

2.1 Open the little fuel chamber. Carefully. It can be happen that the gasket will become cracked.
So try to get 2 new gaskets before, for instance.

2.2 Drill out all Nozzles/Jets and give them in a seperate tub for cleaning, later.

2.3 Remove the upper cover of the diaphragm housing. Take out the diaphragm very carefully, because they can stick to the Carburettor Housing by old, dried fuel to the Housing. Lay down the diaphragms to a clean peace of cloth for inspection later.

Take a fuel-resistant tub, fill in new benzin or special carburttor cleaning fluid and spend the Carburettor Housing a bath for a couple of Hours.

Meanwhile clean the Nozzles/Jets carefully, don`t loose one. Take your Time. Perhaps with Ultra-Sonic-Cleaning or with Brake-Cleaner.

Finally take compressed Air to blow out the Carburettor channels from Cleaning benzin / fluid.
Reassembly  the carburettor a build it in to the Engine.

The attached pics shall help you

With kind regards
Anja