Remove the vacuum line at the advance, and plug it. If the idle speed is adjusted right, there shouldn't be any vacuum on the line, but it never hurts to be sure. If the idle speed is too high, the centrifugal advance will start to operate, and that can affect the position of the ball. Try to keep the idle down to 500 RPM or slightly less whille adjusting the initial timing.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!