I bought a TR Magna and it's dual-fuel. There are two switches, one labelled "LPG/Petrol" and the other "Primer" the primer switch is spring loaded, you flick it and it flicks back. How exactly do you use dual-fuel? It's currently set to LPG, how do I switch to petrol?

You should be able to just flick the "LPG/Petrol" switch to petrol to run on petrol. My guess is that the "Primer" switch is used to start the car on petrol when the main switch is set to LPG. Although modern dual fuel systems start the car on petrol and automatically change to gas, the older systems didn't have this feature and had to be switched to petrol to start and then switched to gas.

Flick the switch to petrol, I'd imagine?

Although, be careful, some of the mixer set-ups that I've had, if you flick it too slowly, when in traffic it'll stall. I prefer to switch it over before starting or at full throttle so there's no delay in the fuel getting to the engine - if that makes sense? :)

Primer switch is used to pressurize the gas system if want to start on gas.
Hot days and hot engines are good to times start up with gas, if the engine is still warm, you might not need to prime it.

Normally the gas is fed into the intake from a diaphram mechanism actuated by vacuum from the intake manifold, this feed only work with manifold vaccuum. Priming will overcome this so it will start on gas.
This applies to mixer systems only.

I wouldn't prine the system without proper instruction from the manual. over priming, or priming when hot could lead to blowing up the intake manifold.

Start on petrol til the choke system is finished and the engine is a little warm, then switch to gas.
Turn the motor off on gas, restart a hot motor on gas.
Overnight or a cold engine, flick to petrol.

And Alscall's advise is spot on.

From what I've been able to work out, priming and cranking at the same time is for gas starting.
Don't pump the pedal, backfires are possible.
Source.
http://fordforums.com.au/showthread.php?p=3346809

Just google ' starting on lpg primer' for more info



Does your Petrol/LPG switch have a middle i.e. is it a 3 position switch.

You can usually work out who's system it is with sticker on the converter ( black box water and gas hoses) contact them for a manual.