My uncle's 1999 GMC 1-ton 7.4L big block dually doesn't want to start and run off of gas since he's rebuilt it. My grandfather has experimented and got the engine to run off of propane that's getting dumped into the intake at the vortech spot. He's replaced the fuel pump with no change. My grandfather and uncle tested the injectors by checking their resistance and whether the fuel rail pressure went down when the injector was fired. They determined that one injector is likely dead, but that shouldn't keep the whole engine from starting. Someone suggested to my grandfather that the timing is off by a tooth or 2, so they repulled and restabbed the distributor with no change. An OBDII code reader/scan tool reports that timing is around 19 degrees advanced, which people have said is too high. He's about ready to hand the issue over to a shop. Anyone have any idea what's wrong? Thanks for any help.