87 blazer tbi - no power

[bowtie5]

I have an 87 tbi 305 blazer. It seems as if the computer isn't controlling the timing correctly. I have replaced the EGR valve, the throttle body and injectors, plugs, wires, cap & rotor, and even checked the timing chain under the cover to see if it jumped. If I set the timing following the underhood sticker at zero degress with the timing plug at the firewall disconnected, I get a very very sluggish ride. I thought the computer would advance the timing to the best performance without any knock, but it doesn't change. If it put it at 10 advanced or so and plug the connector back up it will stay there also. the computer won't retard it back to zero, as i've been told. To get decent power I have quite a bit of spark knock. any ideas??

thanks