My 1980 911SC has vacuum advance and vacuum retard hoses. I have been disconnecting both hoses to set the timing (with timing light). At idle, the vacuum retard hose has about 17 inches of vacuum while the vacuum advance shows 0 vacuum. If you don't disconnect and block both hoses, the timing will be over advanced. By the way, Sears sells a timing light that has an advance feature that lets you "adjust" the strobe light and directly read the advance off a scale. Don't know what they cost now (have had mine several years) but they seem to work well. You can rev the engine to various rpm levels and take readings to get an approximate map of your distributors timing advance.
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