Hey good job figuring petcock wasn't getting enough vacuum. This is out of Honda manual, but same idea.
Check your hoses on carbs. Make sure they match this layout.
Note petcock should have dedicated vacuum hose that's not shared with anything else. Make sure there's no cracks in hose and both ends securely clamped.
Measure how much vacuum it actually takes to open petcock. It might have leaky diaphragm and needs rebuild.