Have you disturbed the high tension leads lately? If so, almost certainly they are crook. LPG requires a higher voltage for the spark to jump the gap at the plug. Normal operation is OK, but as things wear (plug electrodes wear or bend open) or the insulation deteriorates, things go from bad to worse very quickly. Years ago, Champion recommended that the plug gap for LPG operation be closed down by 0.1 or 0.2 mm. Also they recommend a colder grade plug for LPG operation. On my Pajero, I finished up importing some plugs which were not available in Australia (they were much cheaper too, but I try to support local manufacturers or suppliers if they are reasonably priced. Eventually I finished up with Iridium plugs because they had the best spark characteristics.

It is important to run the high tension leads exactly as designed, otherwise they can short to earth (misfire) or short to another plug lead (backfire). Either fault is not good. Once you get regular breakdown, the lead is damaged for good. You can check (to a limited extent) the leads by lifting the bonnet in a very dark area and running the engine. If the leads are shorting to earth you will soon see them.

I have a friend who has a 100 series Landbruiser which he runs on gas as much as possible. The gas tune keeps drifting off and every few months he has to get it re-tuned. The mixture seems to go lean as time goes by and this puts extra loading on the ignition system.

To sum up, you must have good condition high tension leads and plugs. The plug gap should be closed slightly to help ensure that the spark jumps the plug gap, and not goes elsewhere and you should have plugs one grade colder that the recommendation for petrol.

Due to the different burn characteristics of petrol verses lpg . lpg causes backfiring as it ignites too early for the timing set for the petrol only ignition. So any poor tuning of the motor will show up as poor running and back firing on gas.The timing set for Gas fuel has to be a compromise between both fuels as LPG fires a lot earlier than petrol.(before piston gets to TDC) causing backfire. Motor under load advances the timing more so makes the ignition even earlier on gas as the piston is still on the intake stroke with intake valve open so the firing goes back out the intake as a bask fire .So ignition timing on gas needs to be set to fire later in the ignition cycle.
lpg ignites much faster and hotter at about 470deg and tapers off quickly ,where petrol ignites later and slower at around 300 to 400 deg but burns longer. So you need to make sure the motor is tuned correctly for both fuels.

Gary

Replace the distributor cap, leads with high quality leads and rotor button. Next, check the tappet clearances, a valve being held open slightly will not ignite an air/petrol mix because it is heavier and the fuel is introduced spate to the air. With LPG that perfect explosion mix is waiting in the manifold for anything to ignite it. A plug sparking while the inlet valve is open due to lead or distributor faults cause a cross fire, or the combustion leaking up past an inlet valve riding open when it should be closed.
There was a completely different cause in the early model Holden 253 engines. The exhaust cam lobe would chew off and the exhaust valve would not open far enough to dump the combustion pressure trapped in the cyl when under load. The inlet valve would open again and that hot burning combustion would roar out into the intake manifold and very violent backfire was the result.
I've seen it with broken valve springs as well, so something else to check out.

T1 Terry