Just put another engine into my civic it sounds good but idles high at 1750 rpm. When I try taking it past 2500rpm it hits what sounds like a rev limit. It will rev past 2500 if you give it some gas as soon as you turn it over it will rev up but after about a second it dies out and goes back to limiting. Have gone over the loom to check all the plugs but everything seems sweet. Spoke to a mate who had encountered the same problem and said it was a sensor or something but he can't remember what.

Replace the Ignition Control Module:

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