3.9 V8 Problems

I have a 3.9 v8 range rover classic on a H reg wich has been converted to lpg. Iam having a few problems with the car. Firstly it stoped running on petrol, i have replaced the fuel pump and still no difference so must be down to wireing or a relay. Since then it has been run on lpg only, however recently when you push the accelerator pedal down more than halfway it backfires loudly like a gunshot almost and it sounds like it is coming out the airbox rather than the exhaust.

The engine still runs sweet the rest of the time and sounds like a v8 should.

If anyone one here could shed some light on the situation it would be much appreciated as i cant think of anything apart from the lpg may need tuning?

Thanks, Callum

Try fitting a high qulity set of leads and new plugs, lpg needs a good spark, if any HT parts are breaking down it will run like a sack of poo!!!

With regard to the backfire problem ,another thing to check is that there are no splits in the air intake hose from the air flow meter to the plenum chamber , as it messes the mixture up ! Most applications have an anti-backfire flap just after the air flow meter ( to protect it), which on mine jammed in the open position after one of those backfires !I had to remove it by way of two screws and straighten it back out to allow it to swing freely. Also , once when I let the gas get really low, I opened her up to overtake a tractor and it did one of those backfires . This is because the mixture is too lean. If you have the efi v8 then there will be a stepper motor connected between the gas vapourizer and the plenum chamber, which automatically adjusts the mixture of gas/air when on lpg. If this is faulty ,it wont respond to sudden acceleration. When mine was faulty , I fitted an older type manually adjustable type that usually goes on carb. model systems , while I sourced the correct stepper motor. Hope this helps.

AS in previous posts, make sure your ENTIRE ignition system is good. Not just leads (and you don't always have to go for the best. I have been running Halfords rotor arm and cap for about 14,000 miles now with a problem) But I would at least lucas items generally (i couldn't get them at the time.)
also check things like the dizzy amp, earth connections, air filter, air flow meter gas setup etc. I have 2 classics on gas. one used to back fire until I serviced the engine. Then ran sweet there after.
the other which i had on the road for all of two weeks started to back fire. Turned out the dash mounted switch box/control box was faulty.
On the petrol side, have you got petrol going through the fuel rail, injectors and into the cylinders?
If not check ALL electrics including ecu