Help!!!!! My v8i/lpg will not start

Hi all hope someone can help me out on this Ive got a 1991 v8i that has got a lpg convesion all was well and started first time every time untill I did a 2" sus lift,since I finished it has not started at all it will turn over and I have checked that there is a good spark but it still will not go.I know that lpgs allways start on petrol then switch to gas but what stops the petrol from feeding when on gas? how do I check if petrol is getting to engine? and if not what do I do next ?

Cheers Bob