Can you hear the throttle microswitch when it hits the endstop? I remember once not getting a response when grounding the test point. I figured it was a good time to clean the channel which the idle adjust turn screw "pinches off" to control bypass air at idle when the throttle is closed. Remove the air duct between the Air Mass Meter and the throttle housing. I turned the idle screw all the way in, counting turns, then backed it out. Although it didn't seem to want to come out, I left it backed out most of the way, and then blasted throttle body cleaner through the small duct that the screw runs into. Clean around the throttle valve also, but be careful not to get cleaner down its shaft into the microswitch at the rear end. Replace the duct.

When you're done, turn the idle adjust screw all the way in, and then back it out by the number of turns noted earlier. Try re-setting your timing, and grounding the test point again. If you get no reaction, I would suspect your Idle Air Control Valve, or maybe there's an issue with your ECU (Electronic Control Unit).