I Just bought a 1991 Honda Civic SI with 1st gen B16a engine, S1 transmission, off brand short ram intake, ECU I believe is the one from the Japanese swap (Integra), and brought it to the required emissions in CT to register my car for permanent plates.
During both times I brought the car to the independent garage for emissions testing, the test employee was unable to maintain a constant speed of 25mph +/- 2mph on the dyno as the speed of car kept dropping, thus she had to abort the testing short of completion since the computer requires a constant speed to complete the test for a certain period of time.

When driving around town I notice that at 2000-2300rpm there is a stumble of some sort, the car gradually decreasing in speed all by itself around this rpm band even with the throttle at the same exact position, which is the rpm range indicated on the aborted emissions test sheet. If I give the car more throttle and get past this rev band the car feels fine and the powerband is right all the way through the rev limiter. Off the line and up to this engine speed the car feels fine also.

The car has had a cap kit, new water pump, thermostat, plugs, wires within the past 15k miles when the engine swap was put in.

I'm not sure if the O2 sensors are what came with the engine during the swap or if they have been replaced.

The ground from the valve cover to top of radiator support look kind of eaten up.

The idle of the car is fine, doesn't jump and usually is around 750-900rpm when up to operating temp. The vacuum lines seem to be hooked up fine.

On a Mazda I have, an aftermarket intake has given me a boggy powerband at certain rpms, is it possible that this air intake is causing problems, and I should get a stock EF9 airbox from hmotorsonline?

Any other ideas what could be causing this odd powerband issue?

If this problem might be O2 sensor related, what O2 sensor would I need and where should I get it? This car is OBD0

 

Is your check engine light on, are you throwing any codes? Has it ever done this at different RPM's or only in that range? I've experienced bad O2 wiring which caused the O2 to not work properly. The symptoms were very similar. Your setup requires 2 O2 sensors, both to be wired up properly to function.