100 Series dual fuel problems

Gabby
Firstly, there is no need to rev the engine to a high degree, except when checking simulated elevated road speed, which isn't real high revs anyway. The fact it blows the airfilter apart is a clear sign of running lean, and lean burn of gas causes long duration of the burn and is still alight when the intake valve opens for more, therefore igniting the intake air/gas stream and blowing things apart. Seen it happen on EFI Nissans, especially at changeover from petrol to gas running. Your problem may be a sluggish gas converter not acting properly.
The leanness may also be caused by an airleak into the intake system from a split intake hose or failed gasket of parting flanges of the throttle body. I would look for hoses first though, thorough check needed.
Any exhaust fumed detected which enter the car are carbon monoxide killers and poisoning you. ie, gassed to death.
Either the exhaust exit at the rear is not placed properly and/or the rear tailgate seals are not sealing and allowing exhaust gas to enter and be drawn into the cabin when the extracting effect of windows down, is happening. That is, unless there is a clear and detectable leak somewhere along the exhaust system beginning at the exhaust manifold gaskets and further rearward. Even ANY mechanic with a low IQ should be able to find that in under a minute. If you place a rag firmly over the exhaust exit to seal it, any leak point with hiss and the engine should stall by being constipated with no flow.
Are the fumes petrol fumes or exhaust fumes, one is fresh, the other is burnt. Perhaps the constant use of gas and the intake explosions have damaged the intake system, possible causing a hose or gasket damage as mentioned.
Ideally, when changing from petrol to gas and back the system should slightly dual fuel for a second or so, to prevent leanness occurring and subsequent damage from explosions. In early Falcon Taxis, the leanness and load bang used to blow the LH headlight out of the vehicle and onto the road.
Sorry don't live in Melb but plenty of reputable and capable people are there.

Gabby
An after thought. If the engine has been serviced by Toyota or most mechanics, they probably have NEVER checked the valve clearances, ie Cam bucket to cam clearance. Very common to avoid it. If used on gas the lessening of the valve clearances will be faster than on petrol and if the clearances are small, or nothing or even beginning to hold the valve open, then combustion gas will be squirted back into the intake system which is full of gas and air mix. That will surely give the symptoms you are experiencing and NO amount of gas expert work or adjustment will alter it. Only a mechanic who restores the correct clearance through cam bucket shim adjustment will fix it. This is one thing which has to be correct for the engine to operate properly when on gas or petrol. It may be worth having the valve clearances checked if no evidence of the cam cover ever being off is detected.

Hi Gabby.
I would like to help here having owned two gas powered Land-cruisers and when you are saying you have had back fires this usually points to old or faulty spark plugs or spark plug wires needing to be replaced. If this happens some times it can damage the throttle body flap(but not necessarily. That needs to be checked along with the mass flow sensor. As the last member has pointed out the valves on these motors need to be readjusted from time to time. These are adjusted with changing shims . The valve seats wear more when using gas.
I hope this helps.
Laurie.

LPG systems are much harder on the high tension electrics than regular petrol. They require a higher voltage for the spark to jump the gap at the plug. Champion recommend that the plug gap be reduced by typically 0.1 to 0.2 mm ie instead of 1.2 mm, reduce the gap down to 1.1 or 1.0 mm. They also recommend using a heat range of plug one grade colder than the recommended plugs for petrol engine. All this helps to ensure that the spark jumps to earth at the required point - at the spark plug electrode not elsewhere.

I am not totally sure about the 100 series, but certainly the 80 series engines had 5 mm diameter spark plug leads. So did the Pajero engines. After a while, the leads become brittle with heat and age, and when you remove the lead to replace the spark plug, you can easily upset the resistance of the leads as well. End result is that you get leakage to earth or to other leads. My Pajero was very touchy about HT leads. I found that I had to be really careful when running new leads to ensure that they did not touch each other, or sag so that they touched the engine frame or the rocker covers. I found by experience that the best leads for my Pajero were NGK - they lasted the best and also were numbered so that the leads were the exact length required for each plug. Other brands had longer leads and they shorted out either to each other (cross fire = backfire) or to earth (engine missing at times). NGK were the best as far I was concerned.