V8 disco

hi every one im new on here so not to sure wot the crack is but i need some help !!! iive had 1 or 2 landies over the years starting off with a series 3 with a 2.1/4 petrol then a 3.5 range rover then another 3.5 then a 200tdi disco and then another 200tdi (witch is going on ebay as soon as i get this one to run how it should ) anyway ive just bought a 1994 V8 petrol/lpg and its back firering through the induction not to bad on petrol but big time on gas ive changed plugs /leads and dissy cap could it be the lambda sensors ? cheers bish.