1989 Renault 5 Automatic (1397cc) Kickdown/Carburettor problems

[Renault 5 fan]

Looking for some help / advice on a Weber DRT 32 carb. Vehicle is a 1989 Renault 5 Automatic (1397cc) with 29,000 miles.

Fault occurs when the kickdown function is used (accelerator pedal is fully depressed). The gearbox changes down a gear as it should and it feels like everything is working as it should but then when you ease off the accelerator revs continue to climb. Rather scary first time it happened but calmly slipped it into neutral and the revs raced for a couple of seconds then dropped back to idle.

Tried it again many times, under controlled conditions, and exactly the same things happened. The car continues to accelerate under load until I shift it into neutral then the revs race for a couple of seconds then drop down to idle. I then slip it back into drive and car operates normally.

I have had the carb out and given the two butterfly spindles a good check/clean/wd40 and they are not sticking. The accelerator pedal is certainly not sticking. There is an electronic sensor switch that is triggered by the kickdown and this is wired to a control unit but I am not sure what this does. There is also an accelerator pump lever that is triggered by the kickdown linkage on the carb and this looks to be operating normally.

The Haynes manual for the car refers to "additional enrichment provided by a pneumatic device" under high load or acceleration which opens a "valve" to allow additional fuel in to enrich the mixture. Could this be sticking? and gets released when I slip the gear to neutral?

Any help greatly appreciated.

Thanks