Pete-B said:

Looks like your engine is a bit of mash-up of parts like mine. My engine block is also a 24G suffix B engine (which originally is a 90/110 detoxed engine). But the rest of mine is from a non-detoxed engine so it has less emission control stuff on it. Yours certainly has the RRC air filter as the standard Defender one for a 1989 vehicle has more pipework. If anything your's look more efficient in terms of airflow but may require different needles in the carbs to benefit from it.

It looks like your coil might be mounted quite low do so could be close to the exhaust and getting very hot which won't help. They should be mounted high up on the wing next to the windscreen washer bottle.

What's to the right of the radiator? It looks like it's part of an LPG system which is why I asked.

In tuning an engine the carbs are the last thing to look at. Everything else needs to be right first. I'd recommend a service as a starting points. The spark plugs should be a pale beige colour and yours certainly look black. Plug gap is 0.8mm. A new distributor cap would also be worthwhile and leads as well if your's look old. Go for OEM quality if you can.

While the spark plugs are out, have a look into the cylinder bores to see if there is much carbon build up and whether it is worth taking off the cylinder heads for a deck. Mine was quite bad.

The timing can be set quite a lot more advanced than the 'correct setting' and that does seem to help. Des Hamill’s book of V8 tuning suggests over 12 degrees static advance but I found that a bit much. Easy enough to try different values to see what works for your engine. Check that the timing advances when you rev the engine to make sure the mechanical advance is working. I had to replace my distributor because one of the advance springs had failed and it was generally worn so, even when idling, the timing was moving all over the place. It’s also worth checking that all the cylinders are firing as the engine will still run smoothly on 7 or possibly even 6 cylinders.

Check all the vacuum pipes from the carbs and manifold have a good seal. This includes the pipe to the brake servo and the servo itself (if the brakes work well then these should be OK).

Check the fuel pressure if you can. Low pressure could be down to a worn fuel pump or fuel pipe blockage. The higher ethanol content in petrol nowadays can cause the rubber in old fuel pipes to breakdown which can then block the pipe restricting the amount of fuel getting to the carbs. I definitely had this problem so replaced most of mine but need to replace the remainder. I also put in a new fuel pump because the pressure seemed low at less than 2 psi. That didn’t seem to make any noticeable difference though.

Also check the compression. Mine were all fairly close to 150 psi which I think is a bit low but they’re all within 5% of each other which is good.

The camshaft will start wearing out at 50,000 miles and likely to be knackered by 80,000 miles. If you take off the rocker covers you can just about see down to the cam with and with a torch you may be able to see whether the lobes look worn or not. But even if it looks OK, it is still likely to be worn at high mileage. You can run the engine with the rocker covers off to make sure all the valves train is operating. When I did this I noticed that one of the inlet valves was barely moving and that was down to a completely worn out tappet. If the inside of the rockers covers and the valve gear are covered in black sludge then that would suggest that the engine is very worn and hasn’t been looked after. The RPI website has some good pictures showing what it should be like.

Finally make sure all the crankcase breather pipes and oil traps are clear. The carbs will have been set up originally with some air coming in via this route so if there’s a blockage then it might have a small impact.

When I had fixed all the problems identified from my checks the engine felt as though it had slightly more power but wasn’t as smooth. So my final step was to rebuild the carbs with the proper SU kit and then set them up in accordance with the official Land Rover manual. After this it finally ran smoothly. I’m not sure whether the carbs needed rebuilding but as I was taking them apart anyway to set them up I thought I might as well do the full job.

I now get 17 mpg on a long run, even when towing, and it runs more smoothly and feels more lively. There is still more work to do but it is a lot better.

If you decide to replace parts then I can tell you what I did in terms of the camshaft and cylinder heads.

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Pete-B said:

As far as I know the suffix B indicates engines that had the later front cover with the crank driven oil pump and also composite head gaskets (with matching cylinder heads). The blocks are fundamentally the same and can be interchanged.

I would guess that the H suffix on the V5c is a typing mistake.

There isn't a lot of information about the relevance of the suffix so happy to hear from anyone who know more about it.

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