I found mine used to run really well on a 50/50 petrol/diesel mix then about 6 months and 5Kish miles later it started to run like a sackoshite then blew a cylinder head damn near clean off!

Jono,
surely it's unlikely to be fuel starvation, since that would be most noticeable at full throttle?
Since the problem on your car is at tickover or during the transition stage (during which the drillings just above the butterfly are active) my guess is that there's a blockage in the idle system.
It could be worth trying the 'hand over the carb mouth' trick, with the engine running at about 1/4 throttle, to see if the engine vacuum might clear the obstruction...

ken.

Thanks for all the suggestions etc.

I tried the hand over the carb trick this morning, sadly to no avail. So I gave up and fitted another carb. Problem solved

To roughly set up the mixture, do you richen it until it about stalls, and then weaken it by about a 1/4 of a turn?

Hi Jono,
good to hear it's sorted.
Idle mixture adjustment heads in the opposite direction, leaning it off until the tickover falters, then back the mixture screw off (anticlockwise ;o) until it's even.
Usually around 1/2 a turn is needed.

ken
( You won't be binning that other carb, since it's probably something fairly straightforward that's involved?)

Thanks Ken. I'll have a proper look at it later (although it seems to be running happily enough). I might even take it to the MOT station and do it properly.

No, I certainly won't bin the old carb. I guess it needs a thorough clean-out, although I might take it to the International to see if any knowledgeable eyes can spot anything else wrong with it.

Good to know you've got it sorted Jono.

Hi Jono.

Glad it's sorted - very frustrating sort of problem.

Classic symptoms of either a blocked idle jet (which you seem to have checked and discounted) or an inlet manifold leak - it only needs to be tiny.

I had a similar problem when I collected my grey Charlie - half way home the throttle started behaving a bit like an off/on switch; it wouldn't idle and you could feel it completely 'die' when the throttle was backed off. Squeeze it a bit more and suddenly it would pick up and run fine. When I finally arrived home, I found the carb securing bolts were loose...

Worth having a good look at the underside of the carb body where it sits on the spacer?

Hi there. Thanks for that. As it's all fine now (and I had previously removed, cleaned and refitted the carb, to no effect), but was fine with the new carb, I reckon there's either still some hidden muck in the carb, or that something's gone wrong in the carb that I've not spotted. Thanks for reminding me about it - I'll probably take the old carb to the International to let some knowledgeable eyes check it over.

It's always good to find out the real cause!

Cobblers. This car's been running fine since I changed the carb in August, but this morning it started acting up again. The first sign that something was up was spluttering in the exhaust when freewheeling downhill, and then when I got to a junction the engine died. It started up again immediately, but now it refuses to tickover. I've removed/blown out the idle jet and the little filter above it, and tried the hand-on-carb trick. All to no avail.

Any ideas?