I just swapped out a B18A with a B18B…  
After the swap, the engine fired right up, but the engine hums.
At idle, the engine runs @ about 1500rpm and drops to 750rpm, and then back to 1500rpm…
It will go one until I turn the engine off…  
Can anyone help me understand what I did wrong?
I used the fuel pressure regulator that came with the B18B.  Could this be the problem?  Anything I need to tune in for this B18B engine running with stock B18A electrical system?

Any help is appreciated.  

your integra is obd0 where as the b18b is obd2.. I have never seen people do this swap but it sounds like your computer is starting to re learn ita self..  

Does that mean there is nothing I can do to adjust any setting or fast idle.. etc?  What about a new computer?  

Double check the intake manifold and see if there are anything that needs to be plugged up.  

try bleeding the air out of your block  or radiator.  

Alright, I think I got this pegged!!!  99% sure.
If anyone can confirm that B18B has differnt CAM shaft then B18A, I will appreciate this.

The problem statement is that after I swap out the B18A out of a 91 integra with B18B engine (I think it is from a 92 or 93 car) imported from Japan.
I used my original intake manifold because B18B intake  manifold is not emission control in Japan.
After the swap, the engine "hunts".  It got stuck in a cycle where it rev between 750RPM to 1500RPM when the engine is warm.

I cleaned the EACV, no help.
It is not the O2 sensor, because the check engine light is not on.
The code on ECU reads out #7, which is the TPS.
Instead of replacing it, I adjust my idle speed screw.  Sure enough after a couple turns, the engine rev. comes down and it drops to about 750RPM, and the "hunt" stops.
I cleared the ECU code, and will drive it for a few days to see if the ECU will spit out the same code.  

Will keep you posted.

If I am wrong with anyone these, please feel free to post your thoughts.  

i believe the b18a and b18b camshaft has the same profile but the b18b head has a better flow rating than the b18a head wich is restrictive on flow.. as for the tps screw sounds like you got it to me! my only other suggestion would be if it still surges check the vaccum lines one may have a small crack or hole affecting the pressure or you may have one mixed up  

Alright! here's your lowdown...All B series engines are interchangble..meaning all of the parts can fit on all B blocks.  However, you may need to double check and I do recommend for you to double check the specs for each and every part that your going to use.  
Now, for your problem, I pretty much have the same problem before but not a swap. Instead, a replaced head gasket. Your problem are the following:
- ECU - check if you have the right specs for your ECU (ie. OBD0 are different from OBD2)
- Timing - check your timing.  Sometimes, your timing are off by maybe 1 - 3 degrees.  Couple of ways to do this...A) check timing with your timing gun you should be able to see the white timing mark hitting the same mark on the engine itself.  B) if this is correct and you still have idle problems then your cams are not properly aligned.  Your probably off by +/- 1-3 degrees (check your manual for the right specs).
- Throttle body - your throttle body may have a problem.  A leak or a surge might be the cause of the problem. Your IACV or FITV or MAP Sensor may not be working properly. If this is the cause after the above two were done then you may need to overhaul/rebuild your throttle body.  

  I hope your problems get fixed. Honda B series engines are not that complicated to fix yet if you don't know what you're doing and if you're so in a hurry to do so then your gona have a much bigger problem.  Please read your manual and confer/concur with a specialist (ie. honda/acura techs) for further help.

LETS KEEP ROLLING!  

negative don p. not all b series parts are interchangeable... for instance vtec head components will not work in non vtec onless you doo the full head swap also some internal piston crank and rod combinations wont work due to different rod lengths and widths.. vtec water pumps will not work on non vtec motors they have more teeth on the pulley than non vtec so always make sure you get the right parts like that.