intake manifold

I have a 90 civic si with a 95 1.6L SOHC V-TECH in it. I changed from a stock maifold to an OBX air intake manifold. The problem is that the rpm's stay at about 3500 when I start it. It's not the cable I already checked that. This is my first honda I buy and I'm lost ,what should I do? Is there any tuning I must do after I bolt it on?

adjust the throttle cable, its too tight.