First thing to look at is to see if there's gas in the tank.
If there's no gas, then the problem is obvious.
If there's gas in the tank, smell it. If it smells like varnish or some other vile substance, it's bad. Chances are high your carbs will be clogged with this degraded stuff.
If it smells normal, check to see if fuel comes out of the petcock. Since it is a vacuum operated petcock, you need to put it in the prime position (if you have the mod) or apply vacuum to the port on the petcock.
If fuel doesn't come out, you have a petcock problem.
If fuel is getting to the carbs, but not into the engine, you have a carb issue and will need to take them apart to check them out.