Is it a impco fuel LP system? Vacuum shut off or electric solenoid? Vac LP shut off usually is a plugged filter or bad vac line back to mixer. Check for a inline filter at the over pressure valve block. Some had it hidden. Loosen main LP line from tank at vac valve shut off to check for liquid fuel there. I Throw away and replace the top round filter/vac fuel shut off. If it has a Electric solenoid that powers up with key and make sure it stays on during the start cycle. Make sure you see 12 volts constantly or no fuel.
Vaporizer full of oil/tar? Diaphragms have pin holes, high pressure valve bad? Check to make sure also your fuel line from vaporizer to mixer isn't rotten at one end. Vacuum is key to get fuel out of the vaporizer to mixer/carb. How many hrs? Poor vacuum could be a plugged intake,gaskets, leaky valves. At least you know it run on spray.