Hi,

The s2 fitted with mfi, does not have a adjustable fuel regulator as efi,
the fuel pressure can be played with through the fuel pressure actuator
connected to the metering unit, could be cold start valve or thermo time switch that runs the cold start valve, fuel pressure related which you need some decent kit to test primary and differential pressure hot and cold test, for KE system,
to see what is at fault ie regulator/ fuel pressure actuator/ metering unit(fuel distributor)
you could also have a bad coolant temp sensor which tells the ecu one of the ecu's what temp the engine is and it uses this as info for running the engine, loads of things really, you need someone who knows the system,
to check it out,

Regards Dave R.