93 4.3L timing

I think I have a timing problem. After I timed the truck, I reconnected the advance wire and looked at where the timing was. Then I hit the throttle to see where the timing goes. It came all the way back to 0 and then some. Is this normal? I have no power at low rpms when taking off from a stop but ok power at higher rpms when already in motion. The motor has 190K with no major rebuild. Can anybody answer my question about how the timing is calculated? Maybe one of the sensors went bad. Thanks, Hoss