When I'm driving uphill in the Red Special, when the engine begins to labour I can make it pick up speed a bit - a quite noticeable bit - by pumping the accelerator.

Does this suggest I'd improve the engine's performance by putting in a bigger main jet? I think it still has the original factory-fitted one. Whatever that is.

Or maybe I should start by cleaning the carb .....

Dennis,
that improvement in performance when 'over-working' the accelerator pump is a clear indicator that the mixture is too lean.

A lean mixture burns slower, meaning that power output is reduced; once the mixture is brought back to stoichimetric by rejetting, it's common for the mpg to improve, since the same rate of progress can be achieved with a smaller throttle opening.

Fwiw, I haven't yet found a single A series which didn't perform better once the jetting had been changed to suit unleaded fuel...

I discovered that about the mpg a while ago. Mine started using more and more fuel. My garage set the mixture as it was way too lean & it was back to normal.

For some reason the mixture screw on mine is really easy to turn and it seems to go out of adjustment.

I never had the accelerator pedal pumping thing though. The performance was still good. Relatively...

Ian - that "easy to turn" mixture screw is only for idle mixture - it does the job when you are getting CO tested at the MOT - but does bugger all when running under acceleration - the only way you can "unlean" the carb is to change the main jet for a larger diameter one in the body of the carb.....

Ah yeah that makes sense, thanks. We had to tweak it this MOT, it was too rich.
I moved up to the jet size recommended on here, by Ken I think, to help with a flat spot. It did definitely improve it, didn't remove it. But I entrusted it to a guy that seemed to cock everything else up so who knows what went on, lol.

Sorry for the semi hijack Dennis.

When I opened up the carb, the metal tag on it said that it was a 26 35 CSIC. Now the Book of Lies says this wasn't fitted to 2CVs, only Dyanes etc.

First question - can this be right? Or has someone thought that a carb for a bigger engine must be better?

Thinking it was going to be one of the more usual carbs, I didn't look for two main jets and naturally I only found one - a mushroom shaped thing. And it was partly blocked. Not a lot, though. This was on the left of the carb, leaning over and into the engine from the front (ie the driver's side on a RHD).

Second question - is this in fact what ECAS calls the secondary venturi jet?

and third - is the main jet in fact the almost invisible one at the bottom of the other float chamber? I didn't take it out......

and final two - are all these jets marked with their sizes, and which one should be replaced with a larger jet?

This may sound all a bit vague (and it is) but it's cold outside and already getting dark, and I'm not taking it apart again today!

Dyane engine isn't any bigger than a 2CV just a higher compression which is why it had different jets to start with - a 26/35 is the later carb for 2CV - see ECAS site for confirmation - the jet at the bottom of the left hand bowl is the secondary jet and the one on the right is the primary jet and only accessed through the 10mm nut with a small screwdriver - they should have the diameters marked on them - again follow advice in the ECAS site for ideal jet sizes going forward......any blockage, however minor, will give major issues to running and lead to flat spots and misfires etc.

Umm,
Haynes were working from old data, since 2CVs weren't fitted with the Solex twin-choke carburettor until 1978.
They're not the only ones at fault however, since as far as I'm aware an increase in compression ratio does not demand any change on jetting; a carburettor is primarily sensitive to the volume of gas that's passing through it.
There were two versions of the 26/35 Solex carburettor, the earlier one had venturi sizes of 21 & 24mm, the later version was 18 & 26mm.
For the former, a 125 primary petrol jet is preferable when using fuel with 5% ethanol and normal induction, for the latter a 107 primary petrol jet is the best choice.
Here's where the little 'grub screw' primary petrol jet is located, a 12mm spanner is needed to undo the access plug on the nearside of the carburettor body.

Primary petrol jet position, Solex 26/35 carburettor. by slcchassis, on Flickr

My mistake - of course it's a 12mm nut - must stop drinking on a school night.......

Thanks for the help - especially Ken, and particularly for the explanations and the picture. My carb is marked CIT 225 so it's the later 18 & 26 mm venturi type.

I'll order the bigger jets (100 and 107) and see how it goes after that. Putting in a new carb top gasket that isn't torn in a couple of places might help too.