Series 3 Misfire has got me stumped

Hopefully one of you guys can point me right here.
My s3 2.25 petrol has developed a low to mid range misfire with wide-ish throttle openings, imaging you slow down for a junction, nothing coming so 2nd gear and keep going, well it misses like crazy till the revs are back up and it smooths out. The same happens with low revs and extra load, like a hill.
So far I have swapped out the distributor (electronic), coil, leads, plugs, carburettor, fuel pump, fuel filter and fuel hose. I have also replaced all the breather hoses to ensure no inlet air leaks.
And I've checked tappet clearances twice, hot & cold.
My initial thoughts were ignition but then it's worse when hot which made me think fuel.
To say the least it's now driving me daft.
Anyone got any ideas...... Please?

Have you done a compression test, if you don't have hardened valve seats the 'lead memory' might be getting amnesia by now and slowly burning the valves out preventing a good tight seal regardless of tappet settings.
Also check the route of your fuel pipe, petrol may be evaporating in the line as the temperatures rise.
Both easy and cheap to check.

Haven't done a compression test no but I'll add that to the list.
Fuel supply appears good the new pump seems to be belting it out but yes the pipe to the carb does run across the rocker box, simple to change so I'll add a longer hose.

Mine had those symptoms when I first got it but they stopped when I started adding the lead replacement additive to the petrol so I agree with wireman. Did you swap all those ignition bits because of this current problem or had you changed stuff before the problem started? If the latter, try changing stuff back, part by part. I changed my old coil for a new one because it had a dodgy looking terminal, after a couple of weeks the new coil started playing up.

Col

Yes I've been trying to do a bit of fault tracing by swapping things.
The ignition was done a little while ago but I've swapped coil, dizzy & plug leads this weekend to no effect.
Going to do a compression test and see about rerouting the fuel hose on Monday & see what we get.
Cheers

What do the plugs look like? Colour etc? The times you desribe the misfire could be when the cylinder pressure is higher and there's a weak spark. What are the plug gaps?

Have you change the rotor arm?

Hicap phill said:

Have you change the rotor arm?

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Good point, just because it is new doesn't always mean its good, if the brass strip on the top isn't totally secure it could give you your symptoms.

Don't use the crap black ones.
Get a Lucas red one.
Have you replace the coil for a non electronic ignition one?
Is the dizzy cap internal pin intact?

Bob weights in dizzy still working?

Check your timing may be out a bit after the change over?

Plugs are a little on the rich side after changing the carb but were perfect before, just need to get the mixture dialled in a bit better.
I'm using the plugs from Accuspark that have the triple arm around the central post so gap is set but would seem they give a nice fat spark.
I haven't checked the rotor arm itself or the cap but I did swap the complete dizzy out for a different one with no change so going to assume they are all good.
Timing looks spot on at 6', tempted to try it with more advance just to see what happens but I'm going to check cylinder pressures first, comment above about valve seat wear pricked my ears up. I have done about 9000 miles in the last couple of years and haven't been using any lead replacement with regularity so well worth a good look. The problem only started after the truck had been sat for a couple of weeks though, so still not convinced but plenty to work on tomorrow.
I'll post back after I've been through all above.
Cheers folks.

Well I think we could have a candidate, just tried the compression tester and getting 150 on 1, 2 & 3 but cylinder 4 is down to 120. Squirt of oil got it up to 130 but still a good bit down on the others.
Before I pull the head off are there any other tests I can do to check valve seats?

Before you go too mad chasing it down, mine did the same (and I did the same sort of chasing, plugs, leads, disti cap, already electronic points), but in the end when it would not start the truth came out, it was the fuel pump failing. If you search for it you'll see it turned out to be quite common with misfire like symptoms! New £19 pump and it's running like a dream again, and no need for the electric pump ideas I had been having previously. You could also rebuilt your existing pump.

squircle said:

Before you go too mad chasing it down, mine did the same (and I did the same sort of chasing, plugs, leads, disti cap, already electronic points), but in the end when it would not start the truth came out, it was the fuel pump failing. If you search for it you'll see it turned out to be quite common with misfire like symptoms! New £19 pump and it's running like a dream again, and no need for the electric pump ideas I had been having previously. You could also rebuilt your existing pump.

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Already changed to a new pump and no difference other than the fuel filter is full now.

"Already changed to a new pump and no difference other than the fuel filter is full now."

Oh! you do mention trouble starting after a while, which could still point to a failing pump diaphragm.
you mention timing, are you running on unleaded, if so the timing won't be per the book, it be worth tuning be ear - remove and block vacuum advance) and turn disti to get max revs at idle. reinstate vacuum and tighten, also check plug and point gaps are correct.

Those pressures are not that bad, should still be able to run OK.

rob1miles said:

Those pressures are not that bad, should still be able to run OK.

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Not sure where that leaves it then.
Carb, dizzy, coil, plugs & leads are all known good. Timing is spot on as are tappets.
Couldn't find any issues with fuel supply, running out of possibles??

So todays tinkering has shown that it runs better with aprox 10' of advance at tickover.
Most of the misfire is gone but it is still there.
Mechanical and vac advance both appear to be working.
Is it possible that I have the wrong comb on the front of the timing cover or that it was bolted on wrong? I know the engine was rebuilt before I bought it.

The engine would have set up for 4 star petrol. 4 Star had a minimum octane rating of 97 RON. Unleaded petrol (the basic stuff) is around 95, hence the need for slghly more ignition advance. Mine is also set at around 10, but as mentioned I did it by ear rather than with a gun!

Series Land Rover s were tuned to run on 2 star petrol ,90 octane, they would run on even lower octane if the timing was altered, hence the low compression engines, so unleaded, 95 octane or old 3 star is actually too good for it.

Hi Wireman. Thanks for that info! Makes a lot more sense now why they don’t suggest retarding more than they do.