i got a 96 civic cx with a d16y7 and i just got a b16a3 swap done into my car. when it sitting still it tach goes up and drop back down. the mech said it's something to do with the engine vacuum... have anyone had this happened before

 

check the FITV behind the manifold should have small coolant lines goin into them, open it up, should be a white screw like deal, screw it back, sometimes they back itself out.

https://honda-tech.com/forums/acura-integra-6/%2A%2A%2A-how-%2A%2A%2A-clean-your-fitv-aka-fast-idle-thermo-valve-1564019/

 

try adjusting your timimg a bit.. it could of gotten loose durring the swap and moved around

 

my freind had a rough idle problem and it was fixed by adjusting the timing at the dizzy... than it got worse so we looked at the timing belt and it was off a couple teeth.. fixed it and now it runs great

 

search for FITV and IACV cleaning