I completed my engine (D16A6 replaced by SOHC ZC with intake from original engine) swap for my 91 CRX Si.

The engine fires and warms up. (seems to idle a little low, around 500 rpm; and maybe a little rougher than my D16A6. But, the guy I bought the ZC from said he put a bisimoto cam for a little more performance.)

Once it's warmed up, it will die at idle and won't fire without giving it gas.

Turned out to be a vacuum leak at the intake gasket.

Retorqued intake manifold using 12mm 6pt flex socket, 3" extension, and inch-pound torque wrench; three equal rounds: 68, 136, and 204 in-lbs; final = 17 ft-lbs. per factory spec for the intake.
(note: had to pull fuel rail and move clutch cable, throttle cable, PCV valve & hose for access.)

Warmed up, check and adjusted ignition timing.
(maybe, one more timing check to do with "kill ECU adjustment" jumper in place; blue unconnected 2-wire plug near passenger door hinge.)

Basically, it looks like the problem is solved.
Now, I have to tweak the throttle cable slack a little bit (it's actually idling a little high now; factory spec is 10-12mm deflection; a simple one wrench fix).

How did you find the leak? Starting fluid?

I didn't find the vacuum leak. I worked backwards through the stuff I did while doing the swap. I had problems torquing the intake originally. So, I retorqued it.

One of the you-tube videos I watched mentioned using low pressure propane to find vacuum leaks. It's heavier than air and can easily be guided using surgical tubing. The engine will speed up when the propane finds the vacuum leak because the propane acts like adding more fuel.

Starting fluid is used the same way.
Good you got it.