I'm no expert on LPG but backfiring can be a sign of your exhaust system having a leak around the manifold area somewhere. (Perhaps a leaking manifold gasket due to warping of the head?) Under de-acceleration it sucks air into the exhaust system causing a great mixture of unburnt fuel and air to cause a backfire when a spark ignites it.

It might not be your problem but I think that is where I would start looking.

Good luck

hi there,
change spark plug leads with new ones
change spark plugs

tune in engine

should not back fire anymore!

Tony

I have the same problem. Replace your plugs and leads. Did you sort your problem out?

I have a NJ 3.5 on LPG.Had backfiring when I first installed a 2nd hand unit off the same model car.Was told by my reputable Gas man that I had the wrong system fitted (fitted new from a new car dealer) and was running too lean.Had new system fitted and all was well for a while,then backfiring again.I was then told that Pajeros on LPG don`t like platinum plugs.I was using Iridium.The electrode is too fine and burns because of the excess heat in the combustion of an LPG motor.Changed to a good quality
standard type plug(NGK)and no problems since.

OK, here is the information that I have received thus far:

1) LPG will expose problems faster than petrol
2) The plug gaps need to be slightly greater than for petrol, i.e. if the default is 1.0, then it may need to be 1.1 (check with your gas fitter on this)
3) Type of plugs can also make a difference. Am trying with Iridium instead of platinum at the moment, will keep you posted.

Best wishes,

Z

joegigs

I have to agree,

"because of the excess heat in the combustion of an LPG motor.Changed to a good quality
standard type plug(NGK)and no problems since."

All M/C riders in the 80's experienced the same sort of problems when, 4star petrol was took off the bowsers, leaving us with 2 star only. A simple lack of octane produced so many problems for so many people..


LPG is a very hot burning fuel. It makes sense that, a much hotter grade plug would be needed given the intensity of heat of Lpg combustion. Most experimential 4 cyl Jap bikes owners ,-in the 80's,- went from NGK BR8ES-BR9ES, and that solved the problem overnight.


I once had an 86 XF Wagon, I found that advancing the timing 1-4 degrees, eliminated that pinking sound, and occaisional backfire for good, and, also increased performance, when, i was forced from premium to unleaded. I don't know about LPG, though, it is a very dry, hot burning fuel. Have you considered a lubricator, incidently? Me mate was on a long haul once, and siezed the top end of an LPG coverted Pajero, beause of the absence of thence? Anyway, are there any leaks at all, including all the air inlets, vacumn hoses etc,etc?


My 2c

Hmm, OK. I am under the impression that the plugs need to run a smaller gap. This is to ensure you get a spark with a lean mixture. Are you still getting back fires? I ahve blown apart an air box, and dented my bonnet twice... I ahve installed new platinum plugs with standard gaps, but I still get back fires when the tank gets below 1/4 full. I am going to change my plug leads to Top Gun MAX300 leads in the next couple of days. I'll let you know how they go.