Help, no power

[n2963p]

Trying to help my son with his '99 Blazer with a V6. He has just replaced the timing chain, and now it starts and idles fine at about 700 rpm, but when you step on the gas, the best it will do is 1400 rpm. If it were an older engine, I'd suspect fuel starvation, or possibly timing, but the timing can't be changed, and I can't see what's happening with the gas. Idling, there are no codes. With the throttle full open and the engine running at 1400 rpm, the only error is P300, misc random misfire. Any ideas on how to figure out what's really happening since the computer won't tell me.

Thanks in advance.