Just bought 90 civic great car except mid range rpm rough running,after about 2300 rpm she takes off smoothly I tried swapping dist. throttle body ,map sensors from another civic and it still has this problem. I do love the challenge of figuring it out w/o dealer help but am getting discouraged. The plug wires are fine as is cap, rotor, denso plugs,etc. check eng light does come on some times if I really push it (like up a hill) but light is not on all the time.

It's located inside the distributor. Here's what it looks like and costs:

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Here's how to replace it:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-46563-4.html#post265896