To much expensive bits.

We have had the same problem with a few LandRovers It appeared that the expensive sparkplugs don't work on the V8 Try the cheap standard Champion plugs and a good quality standard lead set
with standard amplifier and rotor and cap. On LPG renew these once a year and you should be fine.
Problem with carbs is that they wear the needles and seats within 8.000 km on LPG. After that running on petrol is poor.
Expensive bits are absolutely not needed when running on gas. And I know. I own a LandRover specialist garage in Holland. I have been converting, driving and maintaining RangeRovers on LPG for the last 15 years. Both carbs and injection. And LandRovers from the 109 V8 til the 2000 New RangeRover.
I notice a great miss of experience in the LPG fitters in the UK. Due to the new emissions regulations in Europe and the rest of the world new technically advanced LPG systems are invented every time. But here in Holland we drive on LPG for years. We started with oncontrolled systems. And they work fine. Maybe they use a little bit more gas than a fully controlled set. And maybe you loose a few horses. But compared to the price you have to pay and the problems you have with the fancy LPG kits it is not worth the extra money.
And the same with your friends 4.6. If he uses a plate mixer on top of his Holley carb, he is in trouble. The Holley is a very big carb. And that gives a very poor vacuum above the carb. In order to obtain a vacuum needed to get the gas flowing in the carb at low engine load it is neccesary to restrict the airflow to the carb. Problem with this is that at high engine load and RPM the flow is restricted too much and you loose a lot of power. Tell your friend to take the mixer off and try again on petrol.

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