The idle is controlled by the ECU. However it relies on a solenoid style actuator to operate so the final link is mechanical.
The SPI injector unit is so similar to a carburettor that if you are used to the old technology then the SPI will not be too much of a problem to you.
There are 3 most likely causes (Check / do Step 3 FIRST as this could be the cause)
1) Incorrectly adjusted/seated accelerator pedal cable. Check BOTH ends. I would also completely slacken or remove it from the SPI unit and see if the idle problem still exists. Whilst it is disconnected (engine off please) gently operate the SPI throttle plate from fully open to fulled closed a couple of times feeling for roughness, sticking etc. At the same time look at the solenoid mechanical interface to the trottle plate. 2) Sticking, damaged or bad machanical interface of idle speed solenoid. 3) Air leak on one of the vacuum hoses. (one big hose to brake servo, one small to vacuum advance sender unit for the ECU). NOTE! On my Tipo 1.6 SP I had several of these hoses split where they connect to units. ALSO there is probably a 'T' junction (on the front bulkhead to left of the central black relay and vacuum unit box/cover. This junction is for the Econometer on the dash. This is where mine would split at the 'T' insert was really too big for the bore of the hose. To check for a leak.split at the dash econometer just suck on the end of the hose that goes to it and seal the end with your tounge and wait.If all the above are OK then you will need to dig deeper. We then get into temperature sensors/wiring etc.)
I suggest you does the 3 steps above in the order 3, 1 and 2.
Let us all know how you get on.
Nick /////