Try this shut gas off drain lines of LPG and remove the air hose going into the carb after that get a small rag gas soaked and put over intake and crank see if it will run of gas. If its flooding it will cause this wear it will try and try and try to start and then backfires it sounds like the reg is bad. Those reg Su2k 2 get right. U may want to remove the air mix screw and tighten all the way in and then back 3 full turns I found that's a good starting point. After that get a heater hose tee splice that in and hook back up and with the 3rd fit ur vacuum gauge in and crank see with the carb is seeing for fuel it should not go over 3.5 psi and with the 1/8 pipe tread it should not go over 1 psi and before u try cranking I would check the firing order that's to ez to over see and that will send u for a spin also check fuel flt also don't want a dirty flt