The injectors for single cams are the same as the dual cams (except the Y7) so that is not the problem. At first I was thinking it was mechanical timing, but you said you have checked that multiple times. Are you checking the oil pressure at the front of the head while in vtec or just at idle. You can rev the motor and hold it at about 4k and manually activate the solinoid (jumper wire to battery). See what the pressure is with the solinoid open. Also, you need to try a different set of cams. This is a easy test if you have some stock GSR's. Finally, you may have to pull all the rockers and make sure all the pins are working right. Any loss of oil pressure down the rocker shafts and vtec will not work. Good luck and let us know. Later.