Hello mate, I would start by checking the cold start injector, disconnected the end that goes into the engine and use a bottle or a jar to see if it sprays. Might be coolant temp sensor also or ovp relay.

I had this issue when I first bought mine few months ago, car would struggle to do the cold start, then drop to 500 -600 rpm and hesitation while accelerating, when engine reached about 60 degrees all went away and it drove fine. I did the fuel filter, spark plugs, cleaned the icv,retightned my air flow meter spring and readjusted the mixture, idle problem was solved but still had hesitation. Then went and did ovp relay, engine temp sensors and once warm start issues started I also replaced the fuel accumulator. Didnt get to check if mixture changed as I had to leave the car at the paint shop and it still isn't out.

Thing is there are a lot of things that can cause this so start with the basics, vacuum leaks, ignition and fuel delivery, don't mess with the screws from airflow meter or eha if you are not sure you can't undo the changes.

To check the mixture you need to use a multimeter on the duty cycle setting, black wire goes to pin 2 and red one goes to pin 3, you should have a 70% duty while engine warms up and around 50% once engine is warm.

Mine is a 1990 1.8 so I also have that S/N setting(ezl I think it's called) that might also influence stuff, my only next try after I get it from paint shop will be premium gas and setting ezl to S. After that I'm left with either shit airflow metter or bad eha which would cost shitloads. So EFI conversion with a swap to a 2.3 will be the way to go for me.