SOME months ago I realised a long-held dream and bought a slightly modified 1980 Series 3 Jaguar XJ6 in need of TLC. It has a Chev 350 motor and runs an Impco system with a three-position switch -- gas, off, petrol. I have heard various ideas about the correct procedure for switching between gas and petrol and back again.
BY LAW you're required to bring the car to a halt when you switch between gas and petrol, or vice versa. That's to avoid the potentially dangerous situation of losing power when switching on the road. In any case it's not advisable to be fiddling with switches while driving. Apart from that, the only other thing to do is to not have your foot on the accelerator as you switch over.