hey there, that problem can be cause by 2 things.
in the carb theres a jet, this jet can get blocked. try revving the engine very slowly. if it does build RPMs then that could be the fault. heres a video i made to show the issues i had:
the second thing that causes this fault is the vacuum advance on the distributor. you cant miss the thing, its the large disc shaped object with the pipe going in it from the back of the carb. theres actually 2 pipes on the back of the carb. the small hard to see pipe which controls the choke flap can split, so check for that.
anyway unplug the pipe going to the distributor from the back of the carb now i know it sound a bit grim but suck on the pipe. if you meet resistance like you are sucking on a spring that shoots back when you release the vacuum the thing is in good health. sadly if you can keep sucking on it like the pipe is not blocked off on the other end then the diaphragm has failed inside.
you should be able to get a new one from Ricambio
>>here<<.
alternatively, one from a citroen AX will fit if you drill a extra hole in one of the mounting feet.
naturally check for vacuum leaks, check to see if the brake servo (if you have it) is not leaking vacuum and just listen to see if anything is hissing.
once you have done that and fixed any potential vacuum leaks, you will have to adjust the fuel mixture screw back to its original position or there abouts.
hope this helps!