dont LPG a carb'd motor, its a PITA...
to switch fuels, you have to stop, rev to about 2k rpm, then flick the switch.. as the petrol suply is stopped, you still need to burn the fuel in the float bowls, while also chucking LPG down the carb, this creates a very rich condition for about 30 seconds (this is why you need to rev to 2000 - it prevents it stalling as it boggs). also, as you are effectively running the float bowls dry each time, they are barstewards for picking up crap into the jets..
the only LPG systems worth fitting are the OBDII compliant multipoint systems like the prins system on dads tahoe, they are simply superb, with no noticable change in peformance power or drive, they also self swich to gas when appropriate, and will switch back to petrol if you run out of gas.
these state of the art systems aren't cheap however, typiclay costng about £2500, or if you need the remote injectors like the tahoe does, a wallet melting £2700
