Strange timing issue - '68 318

Wow...Your getting some misleading information !

Running your street car without the vacuum advance is costing you performance,drivability,and fuel economy. If your car runs better without it is a signal that you need some diagnosis and adjustment or repair.
It isn't something you simply disconnect to solve a problem.
Also Always disconnect and plug the VA when setting the timing. Why someone would do it any different is a mystery to me.

Or you could just block it off and throw money down the drain.

Check that your distributor and vacuum advance pod are both in order. After you set your timing,connect it to a ported vacuum (no vacuum at idle) and get back with your results.