My Falcon has just started backfiring while on LPG whenever I accelerate. However, when I switched it back to petrol it runs fine. Whats the deal?

no country told , matters! for sure no USA. KM speeds tell that. GUESS Australia????
down under IRON./? or the exported Austrilia to Africa.???

won't rev out (means RPM limited,means low power from engine. so means engine power is lost.
and NOT:
not transmission slip. (none told A/T or stick?)
good post, lots of causes for any kind of engine power loss
I will not pretend to guess it but in terms of physics yes.
engine has mech problem , cam belt slipping then changing back.
engine low power.
does 4LT mean 5liter engine, or 4L +LT MODEL CAR
The BA sub model falcon? IDK
wiki says

• 4.0 L Barra 182 I6 (petrol)
• 4.0 L Barra 240T I6 T (petrol)
• 4.0 L Barra E-Gas I6 (LPG)

seems there are 3 engines used, day1 sold,
gas Petrol
turbo petrol (gasoline)
and LPG (propane) gas.

deep wiki' me.
182 is 6 cylinder DOHC cam engine with pesky VVT valve timing
chain drive cam +VVT
VVT can stick so there is my first physics IDEA on engine problems.
or stick head valve.

the classic list, (means what is tested in order.) using tools.
1: good engine, means all tests pass,and VVT'
2:good spark ever do a tune up in 20 long years,spark + filters.
3: bad fuel, or bad fueling rates. , fuel pump on lasts legs. sure.
4: egr struck open ,even random they do.
5:;random bad wiring to all that.

that is my best, 12,000 miles away and blind, cant see car engine or transmission there.

run the fault codes
indicates a problem ambient temp sensor not reporting to the ECM to get the correct air/fuel ratio for cold starting

If you're comfy with manuals then i
know few websites that can help you with it. Bcause I've bought it from various websites so I know how each site deals. try www.reliable-store.com Very helpful

I assume this is dual fuel.... LPG and gasoline??????? Is cooling system full? If so, probably plugged filter on LPG convertor. If you are not knowledgeable with LPG, get someone who is. LPG is highly explosive.

the fact that it runs on fuel means that the electronics of the vehicle are all operational. The problem is the fuel selection process for he LPG. The converter for the lpg is prone to corrosion and failures internally so I would suggest that you have an accredited Auito gas specialist look at the gas system

check the solenoid on petrol line from fuel pump to carburettor is switching off petrol to carburettor when switching, you should hear the solenoid clicking when operating. Alternatively you can disonnect pipe fueline at carburettor, turn key ,if petrol comes out of line make sure it shuts off when you turn switch from petrol to LPG

Refer to a repair manual.

Hi dave hill, not really sure on your problem but I have a VN commodore that I purchased that had the fuel pump crook and did the same thing of not starting on petrol. Owner said to hold in lpg button when starting . I got the fuel pump replaced. Mechanic said it was caused by not running petrol through enough and running always on LPG.

Whats the difference between petrol and gas? Dual fuel ? lpg