I recently purchased a 1991 crx si, it has a B16A motor in it, the motor is not the original motor, although it is the did have to B16a to begin with i believe. At first it was only running on 2 cylinders and idle very inconsistently (100-1600) but after replacing the spark plugs it has decided to run on all four and idle at in between 1000-1200. When stationary it will not rev past 4000 but when moving it will head up to 8200 as long as it has no load, although vtec does not engage, i had replaced the oil pressure switch hoping to fix that issue. when in gear, unless the gas pedal is feathered, it will bog out and lose cylinder, the way that i found fixes it is putting it in neutral and reving it up past 3000. I am just curious what might be causing this issue, there is no cel and my ecu isn't giving me any codes.

 

What I would do, is check the oil level is full but not overfilled or low. Then I would make sure there is no leaks, like the vtec solenoid gaskets and etc. You should see a oil trail under the vtec solenoid on the head to tranny. Only takes a few minutes to do that before seeing if your are pulling any codes on your ecu. Its usually under the passenger carpet against the firewall. You could also try a compression check, and also check your rotor and cap fron your distributor.