when i turn on my civic, before it would idle to 3k rpm n gradually go down to 1k within a 2 minute timespan, but recently when i would turn on the car, it would crank for a long time, and the rpm needle would go back and forth between 1 and 2k and then jump to 3k and then back down. while this is happening theres some kick back going on in the exhaust and it sounds like the car is going to die. i replaced the spark plugs thinking that was the problem but i guess it wasnt, any thought on what it can be?

ps- the reason why i repost this is because i have an h22 swap