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I have a 1993 RRC 4.2V8 LSE fitted with LPG

Have had it about 8 months and I noticed the revs dropping when at a junction. Thinking it’s an ignition issue I put in NGK plugs, Magnacor Leads and new Lucas coil, distributor cap and rotor arm. Having had a V8 previously I also fitted an RPi Advance and Retard box to get the timing right when on LPG. Timing was set by a Land Rover independent specialist after fitting the RPi box

For a while there were no problems but after a 3 or 4 weeks the RRC started to hunt badly when the engine was not under load. As it happened it was booked into to another LR specialist to service the LPG system. When driving it over to him the hunting problem got worse.

He had it for several months as he was too busy to look at it. He did find on one of the Lamda sensors was broken, and replaced this. I suspected the RPi A&R box might be the culprit and he tried another distributor (without the A&R box), and it still hunted, though not as bad (presume the timing issue was amplified by the A&R box).

After nearly 3 months I lost patience as the source problem or solution wasn’t getting identified so I went a collected it after getting him to put it back together again. The hunting problem has gotten worse, I presume he may not have set the timing accurately, with the RRC stalling every time I stopped, and the engine hunting when dropping below 1200 RPM.

So, after discussing this with specialist I usually use, a new distributor was suggested as the solution and to get rid of the RPi A&R box. I ordered a new Mallory distributor with coil and finally collected the RRC today. It now runs, doesn’t hunt, revs remain stable. Loads more power now as well

However a new issue has emerged. If I back off the accelerator I get multiple backfiring, if I give it more revs this stops. It sounds like a machine gun going off in exhaust, and whilst excellent for frightening small children is a pain. The timing has been set and it runs sweet on starting, accelerates like a dream, but does make a racket when I take my foot off the accelerator.

I took it back to the guy that fitted the Mallory and took it a spin with him. The suggestion is that it could be a stuck valve caused by it being run on LPG for a long time.

Anyone had anything similar or care to offer advice on what to do next?
 

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1995-2002 Range Rover P38A
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Seems you've suffered the same problem as just about everyone who has had their wallet opened by RPi for one of their Advance and Retard boxes. Cost a fortune, of dubious usefulness and are as reliable as a chocolate fireguard. It's not so much a case of will it fail but in how short a time will it fail. An LPG installer that I know who does a lot of work on Land Rovers has a box of them that he's taken off customers cars that have been bought to him with running problems and 9 times out of 10 removing it has cured the problem.

What LPG system do you have? I assume a single point, but maybe not, but is it open or closed loop? In the hose from the vaporiser to the mixer is there a manual valve or a stepper motor controlled one? Do you have a standard air intake and air filter or K&N or, even worse, a snorkel?

Popping back down the exhaust is a sign of a very rich mixture on the overrun so excess fuel gets into the exhaust manifold and explodes in there, retarded ignition or an air leak in the manifold, irrespective of what fuel you are running on. Highly unlikely that it's a stuck valve as if it was it would run like a dog as you'd be down on compression on that cylinder and the LR V8 runs better on LPG than petrol with no ill effects. My LSE did around 150,000 of it's 185,000 miles on LPG and pulled like a train while my current daily P38 has 290,000 on the clock with almost 200,000 of those on LPG.
 
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