High idel
 

I have a 1991 Rx-7 non turbo I just purchased it a couple of weeks a go and noticed that it Idels high on the start I know about the cold start system but it go to 3000 rpm then comes down to 1500 but never makes it down to 750 and then it seems like it is missing when it is sitting at idel. I did a tune up hoping to fix it and it didn't I don't know what to do now any suggestions