Yeah I've been around cars my whole life and never used a timing light. I didn't even know you hooked it to something lol. The vacuum guage is just to sync the pressure of your carbs.
yes, vacuum gauge for syncing the carbs/cylinders.
But that is not what he said
I remember using a Vacuum Gauge to set the timing correctly. Very accurate way. Eliminates all factory errors.
timing is changed by I guess the CDI/ECU on your 4 stoke motor using inputs it gets from sensors.
you have no idea what it is doing while running at idle or above unless you check it.
and then compare to the specs in you manual..
if out of spec tuning/syncing would be very difficult or impossible