i'm with Pbody on this, when you adjust initial you move the dist. housing and the rotor stays in the same spot. with the mech advance the rotor must move in relation to the dist cap or you can't actually have advance.
not sure why you can see it with the vacuum and not with the mech advance. must have something to do with the pickup coil trigger the timing light...
Maybe the mech and vac together moves enough so he can see it, but just not with only the mech
A distrubutor machine is neat to use. I was going to snag one, but i finally decided tech was moving away from this stuff fast enough it wasnt worth it. Better to put the money into efi, comp controlled timing.
