I just swapped a second gen b16 into my buddy's 93 civic hatch cx. It idles fine until you rev it too high then it starts revving real high and then dropping (don't know how high and low cuz it doesn't have a tach). It sounds like a vacuum leak to me, but i can't find one. The only codes it's throwing are a 21 (vtec solenoid) and 41 (o2 sensor) or vice versa. Any suggestions?

I had the same problem and it turned out to be a faulty o2 sensor.