1990 TR fuel injection issue

first of all check the fuel pressure in the system

 

I'd say that your symptom is more high fuel flow rate rather than high fuel pressure (if you have not measured any pressures yet) -- like from a stuck, or leaking, or misadjusted FD plunger. If you and your Mechanic don't have a copy of the TR WSM you can get one here:

Pages D7-D60 covers the K-Jet without Lambda system and the common components also used on KE-Jet:
https://www.dropbox.com/scl/fi/bpbzkj5c2ytxaedw84qeb/Workshop-Manual-Testarossa-TR-SPC.pdf?rlkey=nirasnm4rmkoa3wputlojmczx&dl=0

Pages D68-D106 of this supplement covers the KE-Jet with Lambda system (and if you have a KE-Jet without Lambda TR just ignore the Lambda stuff):
https://www.dropbox.com/s/18bidr3sgbofytb/Workshop%20Manual%20Testarossa%20Supplement%20Catalytic%20Converter%20D61-D106.pdf?dl=0

IMO, first thing to do is with everything off and at zero supply fuel pressures (i.e., crack the cold start injector lines open to ensure that both systems are at zero supply fuel pressure), and then just feel the motions of the airflow metering plates with a finger tip to see if they (and the FD plungers) both move smoothly, easily, and return to (and are in) their proper rest positions (or if the "bad" bank is different than the "good" bank). Good Hunting!

 

Do as Steve said. Without the correct book for the car and following the outlined procedures looking for the correct specifications this will be a long, frustrating and expensive exercise. There are no shortcuts.

 

if your mechanic not knows this then it is getting time to look for a better mechanic

sorry please to tell you this so directly. this should be basic knowledge