When the cam duration gets real long the idle vacuum is so low it needs the full timing (or will tolerate it!) to idle properly. Th lower the idle vacuum, the lower the idle cylinder pressure at TDC, hence the need for more idle timing. Low cylinder pressure means a slow burn. If you have 260 degrees at .050 or more, try full advance and see. Less than that, I am not sure. It would depend on the intake closing point.