I have an 88 RX-7 non turbo. It runs terrible. I have adjusted the timing, the throttle angle, the mixture and the idle speed to specs. I have installed new plugs, wires and checked for vacumn leaks. Just above idle, at about 1200-1300 RPM's it misses very badly and you can hear it slightly in the exhaust. At other RPM's it acts like it is missing and just has a rough feel. On the highway it's like brand new. When under the hood and slightly opening the throttle it almost sounds like a vacumn leak but I have checked everything I can but maybe I am missing something.

 

Sounds like a vacuum leak.. How are you checking for it? Spray starter fluid around all gaskets, vacuum lines, and solenoids, and listen for the idle to increase..

PaulC