So did you test-ride bike before purchase? Or at least see it start and warm-up fully?
What is rust-condition of tank? Open up cap and shine flashlight in and look at bottom and sides of tank. Or better yet, post photo.
There is tiny inline filter
inside fuel-hose just before carbs. It may be inside end of hose or it may be sitting in fuel-inlet pipe at carbs. It can get clogged with rust and prevent petrol flow.
Might also be good idea to remove petclock and check its filters on its inlet as well.
To test petcock's flow:
1. unplug fuel-hose from carbs and aim into cup
2. unplug smaller vacuum hose from petcock (2nd-one going inside towards carbs)
3. plug in spare vacuum hose to petcock's vacuum diaphragm port, 1-2 feet long should do
4. turn petcock to ON
5. apply vacuum to petcock's vacuum diaphragm port. Use vacuum gauge/tester or syringe. Some people say suck on hose gently (dangerous if diaphragm is torn, you may get petrol in mouth).
6. measure how much petrol flows out in 30-seconds when flow starts