Maybe a bit of crud got under one of the needles.
There's a pretty simple, fast, and non-messy bench test. Carbs upside down on bench. Cap-off one fuel inlet nipple. Attach one end of a hose to the other nipple, put the other end in your mouth, suck to create a vacuum, and hold the vacuum with the tip of your tongue. You should be able to get a strong vacuum and hold it indefinitely. If this test fails, you can narrow it down to one needle or the other by taking one of the needles out and pressing a finger over that needle seat to seal it, and repeating the test. If you take both needles out and press your fingers over both needle seats and you still fail the test, that means one of your needle seat sealing washers is not working.
There's a pretty simple, fast, and non-messy bench test. Carbs upside down on bench. Cap-off one fuel inlet nipple. Attach one end of a hose to the other nipple, put the other end in your mouth, suck to create a vacuum, and hold the vacuum with the tip of your tongue. You should be able to get a strong vacuum and hold it indefinitely. If this test fails, you can narrow it down to one needle or the other by taking one of the needles out and pressing a finger over that needle seat to seal it, and repeating the test. If you take both needles out and press your fingers over both needle seats and you still fail the test, that means one of your needle seat sealing washers is not working.