Its petrol.
I think I have fixed it.
I wired the fuel pump to a three position centre off switch.
The pump can either be:
Connected to cigarette socket (aux power)
Connected to nothing (centre off)
Connected to original wiring
I had to remove the drivers seat to gain access to the plug/socket, and mounted my switch.
The strange thing now, is the motor starts OK when I put the switch in original wiring position.
The fuel pump is not on, so it must be a vacuum sucking the fuel.
Have left the switch in original position, and if I have related problem, can always switch to aux power.