1. Test for vacuum-leak going to and at petcock. You may not be getting fuel because petcock is not opening. When you have hot-start problem, disconnect fuel-hose from carbs and aim into measuring-cup. Crank engine, does petcock flow petrol? You can also
measure vacuum in hose going between petcock & carbs. Above certain vacuum number, petcock will open and flow. Below certain vacuum amounts, petcock will not open. Simple if you have numbers to look at.
2. If fuel is getting into carbs, perhaps it's not enough due to vacuum-leaks. These leaks suck in extra air and causes lean difficult-to-fire mixtures.
Smoke test doesn't work because with engine off, it needs to be pressurised to escape from leak. Meaning you have to plug exhausts and seal inlet to pump smoke in under pressure.
One iffy way to test is run engine and squirt carb-cleaner or propane around all intake joints, hoses and clamps. Leak will suck in carb-cleaner and cause change in idle-speed. But it's very subtle and not easy to catch unless you've been shown the shift before.
Best way to find vacuum-leaks is to seal exhaust
and intake and pressurise it to 2-3psi with compressor. First, plug exhausts with banana, potatoes, wet-rags, etc. Then use fitting that seals off intake and pressurise with compressor to 2-3psi. For dual-carbs, you can make Y-fitting with two radiator-hoses to use one pressure-inlet.
Then spray soapy water around all joints, hoses and clamps. I found leaks that smoke and propane couldn’t.
