I've spent the last couple of afternoons trying to track down the source of a particularly uneven idle on my van but I've ended up right back where I started. I don't really know how to describe it other than roughly every couple of seconds, the engine sounds like its struggling then comes back ok, then struggles, then ok, then struggles.... You get the point.

There is also a lot of vibration just as it struggles, strong enough to shake the whole car. Giving it some revs makes it run smoother, but still not perfect. There is a noise that I can only describe as a whirr/rubbing-type sound that comes and goes regularly.

I'm uploading a video to youtube just now to let you all hear it - far easier than trying to describe in words.

So far I've checked:

Valve clearances - checked
Plugs - checked
Points - checked
Condenser - replaced, no difference.
Timing - checked
Carb - thoroughly cleaned, removed each jet and blasted with carb cleaner and air, gaskets etc seem ok, float level ok.
Idle mixture - jet replaced as old one was bent(!) and mixture set as per detailed instructions that ken posted here in another thread.
Air filter - clean & intact
Engine mountings - checked
Alternator belt - found to be loose, tightened but no difference.

Any ideas? I'm now officially stumped.

Video is now up!

That's galloping a bit, isn't it?
To me, although 'diagnosis by video' is never the best approach, that engine sounds as though the idle mixture is too lean and the timing has too much advance.
So, have you adjusted the closing position of the secondary butterfly and checked for any air leaks in the induction system.
Also, was the timing checked with a strobe?

If i turn the idle down any more, it struggles to stay going. I know its high, but it needs to be.

I didn't adjust anything on the carb other than the idle mixture and a good clean-out. I've never played about with carbs much, as they seem to posses the same pixies and fairy dust that gearboxes do in my mind. Is the haynes section on carbs accurate? I'm wary of using it too much as the prevailing opinion seems to be that mistakes are everywhere.

The timing was set by strobe - sean taught me how to do it last time i was at his. there seems to be a half-tooth 'flutter' in it, am i right in thinking this could be a worn points cam?

Id pull the small lucar off the alternator to eliminate that load.
Id agree with Ken and look for an inlet leak and then do the idle jet (co2) jet adjustment

Take the points cam off and turn it 180 see if it makes it better or worse. you can also set the nose of the cam - they can be a bit off center.

Other things too check would be fuel delivery- pump, hoses and needle valve on the float.

Obvious comment really but - have you cleaned the idle jet too? - along with, and rather than, the mixture screw, if this is very dirty it will make idle very difficult at low revs.....

Clean 12V to coil.
Change coil and HT wires.
Clean the points with WD40 soaked notepad paper.

Could be a leak in the inlet's on the head's or carb mounting,also problem could be in the valve compresion seam inside the head's....

Ok folks, I've finally had some sunshine time to play about on the car again. Rechecked everything and i must've rushed things a bit as pretty much everything was a wee bit out - thats what happens when you try to troubleshoot in the rain!

Valve clearances are now spot on - bit tight before.

Timing was spot on before, but was waaaay advanced when i checked it again today. Either i was tripping when i did it last time or i moved the box when i tightened the bolts up... Sorted now.

Spark gaps were a bit big - adjusted.

Dwell angle was at 50, now set to 53 but has about a 0.5 degree fluctuation at idle and a 4-5 degree fluctuation at high revs. Is this a worn cam? I had a feel of the cam and noticed it has some forward-back play along the camshaft. Is this normal?

Can anyone explain what Sean meant by '...set the nose of the cam...'?

Carb jets are all clean as a whistle but i did notice some wear on the plate attached to the floats that opens the needle valve. Not huge, but obvious. Would this affect the needle valve operation?

I also removed the exciter from the alternator and things got a bit smoother, so i had a look at all the voltage regulator connections and they seem clean and tight. Might this be an issue with the alternator itself?

I'm sure i'll post more questions as the afternoon progresses

If you mean the small metal tab which closes the needle valve ......I had that problem on a motorcycle carb .......A small indentation & ridge had worn into the alloy tab,it was almost too small to see with the naked eye,but it affected the seating of the needle valve [the valve was'nt fully closing,causing the carb to flood] .....I carefully filed it smooth with with one of wifey's nail files and some fine "wet 'n dry" rubbing down paper ....problem cured.
I dunno if it'd work on a 2CV though,this was on a 250cc motorbike carb
hth
T.