Nissan Repair: Nissan Terrano backfiring on gas, liquid petroleum gas, fuel pressure regulator

Question
Hi,
I'm feel like a fool but in Australia GAS means LPG liquid petroleum gas, I'm just wondering if it will still make a difference if I advance the timing and if not what would you suggest?
Thanks
Tracey
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Followup To
Question -
Hi,
I have a 1997 Terrano on gas with a Navara motor in it, that backfires on deceleration. I have had it in a few workshops and (it has been dyno tuned) and most seem a bit puzzled.
The guy that is looking at it at the moment said it may be the convertor but is not 100% convinced.
Apparently he has had a few navara cars in that do the same thing.
Just wondering if you have any suggestions? and whether it is a common problem with these motors?
Thanks
Tracey
Answer -
The backfire is due to too much gas in the exhaust when you let off the gas.  Timing could be advanced to help with the problem as long as the engine does not ping.  Also, you could reduce the fuel pressure and it might be as simple as a defective fuel pressure regulator.  If yours is not adjustable it might be  good time to check yours out with a vacuum gauge to see if fuel pressure changes as it should and if not be sure to replace it.

Answer
Tracey,

Sorry for the confusion.  I am in the United States and we do have LPG powered vehicles.  Your problem is an over rich condition caused by the LPG control not backing off quickly enough when you let off the gas pedal.  This is something that is specific to the conversion manufacturer.  Now, if the system is installed factory I have not had the pleasure of working on one.  I would bet that there is a potentiometer that tells the computer to open or close the gas regulator and that is what needs to be leaned out on the part just short of coming back to an idle.  if you lean it out too much you can damage your engine so it has to be done properly.  If you let a shop that has experience in LPG know they could do this for you.  I would not do it myself since you could melt the pistons if you get the mixture of air and LPG too lean.