Did you set the timing with the vacuum advance still connected? If so, don't -- disconnect and plug the vacuum line before setting timing. Also, if you have your vacuum advance connected to an unported manifold vacuum source, then what you're describing is normal. An unported source presents vacuum to the advance mechanism all the time, even at idle, so it will be adding some advance via the vacuum diaphragm. When you blip the throttle, vacuum will drop initially and can cause what you're seeing.