Well, if it is running good at this settings I guess you can just leave it alone. But there are some criteria that need to be in synch when initially checking the timing is actually set at zero. First is the top cylinder to be in TDC. Second, the zero timing mark on the flywheel should be zero'ed with the stationary marker. Third, the carbs' throttles must be fully shut meaning all carb's throttle arms or levers should be in their resting state (this is usually indicated with the alignment of the eccentric screw slot to that of the cam's pick up point or notch marking), thus the throttle shutters or "butterflies" should be fully closed and lastly, the idle screw must be resting against the block but not pushing. At this point you are mechanically zero but you still need to verify you are electrically zero'ed using a timing light.
Verify with a timing light that the trigger is actually zero'ed. This is done by cranking over the motor with the timing light on top plug wire (plugs removed and grounded), control throttle in stop position and the timing light should indicate the zero mark on the flywheel aligned with the stationary marker. If not adjustment to the timing rod should be made. From here you can set your static WOT timing to 28~30 degrees. Thereafter, you should set you idle screw settings. After setting the idle screw, you will notice the cams' pick up marking or notch does not align with the eccentric screw slot anymore nor is the timing at zero.
In your case, it looks like the carbs are pre-loaded at zero timing which does not seriously affect the WOT timing especially if set to minimum of 28 degrees. The pre-loading can be done with the link rod adjustment (link between cam and the timing tower) without affecting the WOT timing but affects the position of the butterflies or throttle valves. In short, your idle setting is mostly controlled by the link rod rather than the idle screw at the bottom of the timing tower. The difference is that with the idle screw adjustment, you are also advancing the timing in synch with the butterflies while with the link rod, you adjust the butterflies without affecting the timing.
