You will find that all lpg setups always start on petrol for 2 reasons. It keeps the petrol system working, and also lpg needs to be used on a warm engine. During the winter months you need to allow the engine to warm up. Some of the later systems will switch over automatically when the engine is upto temperature. For earlier cars which have no oxygen sensor and a metal inlet manifold the single point system is all you need. Unless you are going to be doing decent miles its not really worth it.