I just picked up a 91 integra with a 99 ls motor, and sometimes when you start it up it sounds like its only firing on 3 cyl. and has rough idle, but after letting it idle for a minute it smooths out and runs like a dream.. What could cause this?

 

Alot of things, bad gas, dirty IACV, clogged injectors, spark plugs, dizzy, wires, tune, etc etc.

Start out at the root. Seafoam the engine, clean the IACV and see if that helps

EDIT*

https://honda-tech.com/zerothread/1575913