15 degrees of advance at idle is WAY too much. if it won't run properly at 5-8 degrees of initial advance you've got other problems. What is the total advance at 2,000 RPM with and without the vacuum line connected? It should be around 30 degrees without vacuum, and close to 40 at light-throttle cruise speed with the vacuum advance connected. It sounds like your centrifugal advance is not functioning and you're having to compensate by using too much initial advance.
Jerry

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