Well I'll start at the beginning. My 1996 civic lx, snapped the timing belt a few days ago while cruising at about 2800 rpm. Of course the valves smack pistons, valves bent pistons fine. I put a different head on it, from a 99 civic with the y7 with only 56k miles. Got it all back together and it runs better than it has since I bought it, other than this idle problem.

The idle problem is a bit odd to me. First fire up it is fine, drive or slowly rev it and the idle jumps to about 2k rpm. Then if I just quickly press the throttle about half way and release it, idle returns to 700rpm until I slowly press throttle, then the fast idle is back.

 

Double check the harness and your intake attachments (IACV, for example), those commonly get buggered up and causes wacky idle issues.

 

Gonna check everything again after work. Finished it at 2am this morning so I could get it driveable for work, so maybe i missed something in my exhausted state of mind.

 

It's very likely.
is the CEL light on?
You may want to attempt to reset the ECU and triple check timing while your at it.
Peace of mind and all.

 

Cel is on but that is due to the lower O2 sensor according to the codes I just had pulled . Just checked cam timing and its perfect, IACV is plugged in, all vacuum lines connected, can't check ignition timing until after work when I can get my timing light.

 

O2 sensor codes aren't always an issue with the sensor

 

Not always, but with the ex exhaust manifold and no cat, it is likely that the code is due to that.