Setting Valve Clearance using Vacuum Gauge

I think I phrased that poorly. I didn't mean to imply the timing of the spark was changing. I meant timing in the broader sense. The valves are opening and closing at a different time in relation to the spark.

the spark is in relationship to tdc of the piston not when the valves open....

changing lash will have a very small change on the opening and closing of the valves...