The issue with this machine I believe is quite simple. The intake manifolds on these machines are originally designed for fuel injection. When they are converted for LPG they quite obviously do not need injectors so they are removed and plugs are fitted into where the injectors would normally go. What happens is, if you get a backfire it blows one or more of the plugs out. that I believe is the issue, check the intake manifold and when you find out which one is missing you need to block that hole up, it's quite simple.