Some might know about LPG!!
It is MASSIVELY overfuelling on LPG, Co should be nowhere near .5%, that is massively too high.
It needs looking at, most likely something simple like a TOP HAT(about £3) seal depending on which system you have.
Any decent LPG place can sort it, so get booked in soon as.
If they blow back hard enough they can blow engine components apart.
Does it have a spark advance on it ?, if so this can also do it if gone haywire, so I would also check the ignition timing as a starter.
tabetha