Easy to check petcock; break fuel line, feed into container; apply vacuum (by cranking engine, or by applying vacuum otherwise to petcock vacuum line); fuel should flow freely . . .
Does using "choke" (starting enricher) make any difference? The device is a little "mini-carburetor" all its own, mixing fuel and air; if the petcock's flowing, oughta start!
A most unlikely event, but you may have re-assembled the carburetor incorrectly. Make sure you've an air-tight diaphragm seal, and . . . the slide spring holder/needle assembly's put together the right way.