Check for manifold and vacuum leaks. The problem with LPG units is that the aluminium used in the manufacture if the unit is susceptible to corrosion so this problem may be best referred to a gas fitter specialising in vehicle LPG applications . IT could be a faulty gas unit itself in which case you will only be allowed to have it worked on by a qualified gas mechanic.

Sorry sid this is to late to help you but you would have had a burnt valve which is common in dedicated gas vehicles