I just swapped out a 1.6 to a 1.5 in my Honda civic. I am not getting any codes or any engine light on. The car seems to be running okay and idling good. But when I am driving and switching gears mainly in second gear the car starts hesitating like if I switched in to a higher gear but once it starts going then it s fine. I get the same problem sometimes in first gear on take off and through the rest of the gears if I put the throttle to far down it’ll start having that hesitation. But really noticeable going into second gear it seems like if I run the rpms to about 4-5000 before switching to second then it won't do it but anything under 3500prms then it starts hesitating until the rpms get back up then it goes back to normal. I thought that the clutch may be needs adjusting, but I don’t know why its doing this. Other then that the car is running great. Any help would be great. Thanks

Thoughts:
. Sounds like something electrical may be breaking down under load. Personally, I would check/install new NGK spark plugs, rotor, distributor cap, igniter, and coil. It shouldn't be too expensive, if you go to an outfit like RockAuto.com.
. Check MAP Sensor--electrically and vacuum wise.
. Check for vacuum leaks.
. Check oxygen sensor--it may not be properly controlling the duration the fuel injectors are open, thus affecting the proper fuel mixture for a given rpm.
. Check compression, including how it builds, on the new engine.
. Check idle rpm and timing.