What Jets? 670 Holley

'S not clear to me whether you are talking about 50* timing WITH or WITHOUT the vacuum. You want to measure your high RPM advance with the vacuum DISconnected.

Set the timing, no vacuum, high RPM at 38-40 and see where the initial (base) falls.

Some of the factory mechanical advance curves were way too long for performance, but I find it hard to believe a "performance" dist. from Summit would be.