My old 280 had a vacuum operated idle control system (the big metal vacuum element on the left front side of the carb). You should adjust the idle with the screw below this element. If you have the correct idle rev adjust the length of screw operated by the element. With the engine on normal temperature take the vacuum line off, adjust the screw of the vacuum element until the engine rev is 1600rpm. With the engine running you should see the vacuum adjuster move, it is designed to catch the linkage before the engine stalls.
The solex carb is very complicated, lots of things can go wrong. The idle system is separated from the main system, with separate air channels and nozzles. The electric things on the front turn on the idle system