I had the same problem. I unhooked my ISC and then set my biss from that point. Then I could set my timing. Turned out i had such a boost leak in my TB that forced my ISC to close all the time. so when i grounded, my ISC would be neutral and alow too much air in. hense 2300rpms.
My buddy came over with his timing gun, and was wierded out by the fact that without the ground, the timing was steady. unlike his that jumped around untill he grounded it. I am guessing that since im in open loop all the time, my timing dosent need to be grounded.