TB42?
Dual fuel?

What's wrong with the mechanical pump?

Why do you need an electrical facet pump?

Running the petrol pump constantly when the vehicle is running on LPG helps with switching to petrol quickly while driving on the road, since the constantly running pump primes the carby quickly since the petrol is getting pumped constantly upto the point of petrol cut off solenoid next to the carby.

I guess with a mechanical pump, it happens anyway. With an electrical petrol pump(facet or intank), it obviously need to be wired up correctly to be able to run all the time(regardless of running on petrol or LPG). That's done by fuel selector switch being wired with petrol cut off solenoid near the carby(not the petrol pump itself).

In my case, I wired it(fuel selector switch) with the petrol pump, 'cos I usually don't have the need to switch to petrol from LPG on the road quickly. I wired up a priming push button switch on the dash to prime the carby by running the petrol pump for few seconds when I need to start the vehicle or petrol or when I need to switch to petrol from LPG.