I have a AEM one generation older meth kit which doesnt come with check valve
I bought a mechanical check valve n fitted it

i run it in a Evo III , 2.0, FP Green , aiming for 30-35psi of boost
it did hit 420 on a dynojet with 30psi of boost on 93 octane fuel

i fitted the medium nozzle size by AEM rated for 300-450hp engine
the car is currently misfiring and jerking when the injection starts
i have set the AEM to start at 10psi (backdraw of the AEM minimum 10psi set)

how should i go up with the tune , should i tune it to be leaner a lil bit first or should i go for higher timing first ?

thank you

You might be injecting too much for the airflow you are currently running. Also, the water/meth can cause a misfire if you are running too wide of plug gaps or have a weak ignition.

What AFR are you logging when it comes on?