This is a old problem with engines that have been equipped with radical camshafts. Those type camshafts have long valve duration, high valve overlap, the engine has low manifold vacuum and sometimes will not idle below 900 RPM. The mechanical advance mechanism starts advancing the timing at 900 and is fully advanced by 1100 RPM. The timing advance on engines equipped with radical camshafts should be checked after the mechanical (centrifugal) advance mechanism is fully engaged , meaning at 2000 RPM .
Engines equipped with radical camshafts do not need or use a vacuum advance and require the mechanical timing advance checked at 2000 RPM or more with the total timing advance from 35 - 40 degrees. The RATE of the timing advance can be tailored by changing the advance springs and weights or you can install an electronic ignition system that is designed for engines with radical camshafts.