Destructive backfire

oilyfingers · Post by **oilyfingers** » Wed Feb 04, 2009 8:18 pm

I bought my discovery last year with lpg installed, but noticed the airbox locating lugs were broken. After engine stripdown for headgasket renewal, my brother plastic welded the lugs back into place. Once running & on the road, if switched to gas before warm up it backfires thru airbox. Now said airbox is broken in same place which brings me to the question, can i fabricate a replacement airbox from steel with backfire valve installed with little or no issues?

leepop633 · Post by **leepop633** » Thu Feb 05, 2009 8:04 am

I just used a metal rangie classic air box. Its a round one so you will have to make a mount for it but never breaks in event of a backfire. If your local to Consett, Durham, you can have it as i took the metal one off when i sold the disco. Its got a recent airfilter aswell, lol.

ChrisJC · Post by **ChrisJC** » Thu Feb 05, 2009 8:56 am

Might also be worth trying to find out why it backfires.

Were the valves checked when doing the head gaskets?

Mine has only backfired once in 40,000 miles, and I switch between petrol & gas every time I overtake.

Chris.

oilyfingers · Post by **oilyfingers** » Thu Feb 05, 2009 7:44 pm

I spent christ knows how long grinding the valves in on the heads, so i know they,re good. If i switch to gas before the temperature reaches 1/5-1/4 on temp gauge it will slow down at acceleration from almost standstill, then attempt to demolish rest of airbox with a backfire. Once warm, then alls fine bar the reduced acceleration as it feels like its holding back.

ChrisJC · Post by **ChrisJC** » Fri Feb 06, 2009 8:48 am

Sounds like a water flow problem through the vapouriser then. Maybe it momentarily freezes up?

Also maybe a compression test is worthwhile to make sure the valves are all closing properly.

Chris.

oilyfingers · Post by **oilyfingers** » Fri Feb 06, 2009 3:54 pm

If i lived closer to you leepop i'd take you up on your offer. Thanks anyway. Regards... terry

leepop633 · Post by **leepop633** » Fri Feb 06, 2009 6:50 pm

Cover the postage mate and will bung it in the post for you. Send me your address or post code will do. Thats of you want to but dont feel obliged mate. Its yours if you want it.

mcgill · Post by **mcgill** » Fri Feb 20, 2009 10:15 pm

ive had this lpg 87 3.5 efi rangerover for 12 years,only twice its backfired.but ive a hose,i use as a weak link,and of course the blow back valve....i swear by lpg.runs lovely cold or hot....UNTIL..
i had a head gasket go last month,heads skimmed composite gaskets....i assembled ,drove down the road and BANG..Wouldnt restart on petrol,pulled bonnet,the bottom of my afm had blown off,and blow back valve torn to bits....redrilled the air flow and sealed,remodled the blowback valve.........scareded to death of using the lpg now,not cos of the bang but ,the bloody breaking down........not one air leak, i wisjh i knew what caused it........i really think the compression test is a good idea,just incase i missread the preload on putting heads back....BUT would it run so sweet if valves were open in petrol????????

Post by **Rossco** » Fri Feb 20, 2009 11:56 pm

Backfire = ignition leads

Get some 8mm Mallory or Magnacor's

oilyfingers · Post by **oilyfingers** » Thu Feb 26, 2009 10:00 pm

How much are the 8mm leads you recommend?? Is there anything special about the plugs to run both petrol & lpg???

Shadowcaster · Post by **Shadowcaster** » Sat Mar 28, 2009 7:51 am

oilyfingers wrote:How much are the 8mm leads you recommend?? Is there anything special about the plugs to run both petrol & lpg???

http://shop.ebay.co.uk/items/_W0QQ_dmpt ... &_osacat=0

They are quite expensive, but worth it IMO. You also may want to advance the ignition timing slightly and always use Lucas replacement Dist Cap and Rotor Arm. I use RN11YCC Champions, but only because I got them at a good price.

Cheers Rich[/url]

mcgill · Post by **mcgill** » Fri May 29, 2009 10:56 am

as ive said previously, 87 rr v8 efi,had 12 years,ive had backfires occasionally but just blows a weak link pipe off....(YES HAVE A BLOW BACK VALVE)
ive been running very old champion plugs,box standard leads,and id say its backfired 15 times in 12 years,never a problem.
anyway head gasket went.heads skimmed ,valves reground,reassembled and started iut blew air flow metere to bits....this is back in january...
,came on here,
new plugs and magnatec leads,still did the same...
went looking for air leaks found one ,repaired and all has been well...UNTIL LAST WEEK... BANG! NO AIR FLOW METRE
looked for air leaks,none found.
it now blows back occasionally but only when i switch over...
my old weak link pipe,no longer blows off but it destroys the backfire flap and afm..............any pointers....
could it be a petrol too rich/too lean.....its only on the first switch over...

ChrisJC · Post by **ChrisJC** » Fri May 29, 2009 11:11 am

Too lean IMHO.

Does it matter if you're at idle / light throttle / full throttle?

Chris.

mcgill · Post by **mcgill** » Sat May 30, 2009 7:52 am

2000/3000 revs

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