LPG backfiring issues

[RUMBLE THE 2ND]

Hi my dad has got a 3.2 v6 Isuzu trooper which has been gas converted and it has a nasty habit of backfiring in the inlet manifold and blowing the air box cover off this condition has gotten alot worse recently can anyone help please .Any suggestions would be greatly appreciated.
thanks Tom

[Rossko]

Mostly likely its a singlepoint (aka mixer) type system which invariably cause problems on these engines, the inlet plenum is just unsuitable.

Having said that, something is triggering the backfire.

Begin with an ignition birthday, and don't think you can skimp on it because 'it seems okay on petrol'. Sparks must be in top form for LPG. New plugs, regapped 10% smaller than book, new leads if fitted and a careful inspection of coil packs.

LPG running lean can make it more prone to backfire, so you could consider a tune up on an exhaust analyser. Lean running can also be caused by a blown off vacuum pipe (by the original backfire) or by a jammed so-called anti-backfire flap - like a catflap in the air intake. Look carefull for something like this, most jap engines have a myriad of obscure vacuum pipes. If you find one off, do NOT strap it back on with mighty clips etc. - if the backfire can't find its way out somewhere it gets ever more destructive.

Engine wear and tear increases the tendency to backfire - worn or burnt valves not sealing as good as they used to allows flame into the gas filled plenum. Sticking valve stems will do it - it only has to stick once in a few million operations to go bang. This might be unfixable without an engine overhaul

cheers, Ross K

[RUMBLE THE 2ND]

Thanks for the reply will take a look at the ignition system and see what shape its in I will probably change the plugs and leads for starters and see if that sorts it out. Then ill delve deeper into things if that doesn't fix the problem.
thanks again
Tom