turn off the fuel supply, and crank the truck for 30 seconds or so. if it fires, you have too much LPG going in. if it does not fire, try the trick of seeing if it will run on starting fluid or brake cleaner. if it does not even fire at all, you may not really have spark at the spark plug electrode.
you should only have fuel supply at the "carb" throttle when vac. pulls it in. you have a bad regulator if LPG is pouring out of the hose when it is disconnected.