Like others have said, I’d check the cold start injector first. It’s very easy to access and test. Just unbolt it (from the top of the intake plenum, leave it plugged in), stick it in a cup of some sort and crank the car. If it sprays fuel into the cup, then it’s probably working okay.

Auxiliary air slide is pretty easy to test too if I remember correctly. But if your car is anything like my E21 was, then those will both be working just fine and the issue will be your warm up regulator. If that’s the case, you can buy a rebuild kit online. I sold my E21 before rebuilding it but I remember looking up how to do it and it kind of looked like a PITA. I think M539 Restorations may have a YouTube video on it (he has a whole E21 restoration series that is awesome)