The later models of 2CVs come with a twin choke carburettor with the primary (smaller) being used at low revs and a mechanical linkage opening the secondary (larger) as the throttle is depressed further. Fuel is allowed into the venturi from the bowls by small brass jets (screws with calibrated holes in the middle), the size of which controls the fuel/air mixture. As standard the primary jet is a 102.5 which, for modern fuels, is slightly undersized leading to a slight flat spot in acceleration from low revs. To eliminate this fitting a 107 jet is recommended.
The secondary jet is easily accessible at the base of the fuel bowl but the primary is a bit more complex. The first step is to take the top off the carburettor, with the elephant’s knee (air intake pike) removed there are six flat head screws – one of which holds the choke cable in place – and the top of the carburettor lifts off. As the jets are at the bottom of the bowls they will need any residual petrol in them removing.
Now comes the tricky part. On the back right (as you look at it when standing in front of the car) corner of the carburettor is a 12mm bolt head – this is the access port to get to the primary jet. With that bolt removed there is now a head on approach to the primary jet – shown arrowed here:
The jet has a slot cut in it for a flat head screwdriver which needs introducing through the access port. Fortunately in my tool box I happened to have a long electricians’ screwdriver that could have been specially made for this exact job as it extended clear of all the obstructions in this area and made this job much easier than I was anticipating.
With the jet unscrewed it will need removing from the carburettor. As it’s brass the traditional use of a magnetic pick-up isn’t an option so I found a cocktail stick worked well as it wedges in the middle of the jet without damaging it and holds it just enough to withdraw it.
Fitting the new jet is, of course, the reverse of removal – making sure to reconnect the choke cable when replacing the top of the carburettor.
When everything is buttoned back up check the car starts and runs – it will probably need some cranking to re-fill the carburettor bowls. Then, when the engine is nicely up to temperature the idle will need re-setting.