Stall story

YOU haven't said what make your car is, or how old the system is, but I'm guessing it is old, perhaps an old V8. That was the sort of car typically converted when petrol prices first went out of sight. It's possible the engine needs a basic tune. Air leaks in the system may be hindering the operation of the diaphragm in the mixer, or the engine may be down on compression and lacking the vacuum to operate the diaphragm. I would suggest you take it to an LPG specialist to check the system. If it proves to be working correctly, it can install a primer that will inject a small amount of gas when starting the engine.