Sir my next plan is to change the wires with OEM ones. But i think i am gonna have to take vacuum advance into account one more time.
Today i have been studying some articles about PORTED and Manifold Vacuum lines. I think the one going to the advance plate is from a ported vacuum line. Now what this vacuum line does is that it removes advance at complete idle thus allows increase in engine temperature by late burning a lean mix. This technique was used to improve emissions (somehow-atleast thats what i got)
Now as soon as you increase engine rpm, the vacuum just above the throttle plate increases and engages vacuum advance. This goes on untill you reach WOT where this vacuum is no longer available. At higher revs and open throttle, centrifugal advance is available but vacuum advance disengages.
NOW A HYPOTHESIS
As i have observed and mentioned many times that
(1) the vacuum plate offers so much resistance to rotate. There is not much friction in the plate but the spring in the vacuum actuator is quite stiff.
(2)Also note that when i open the throttle in 4th gear to reach 100kmh and beyond, there are many times i feel that the pick just dropped in a snap. As if you turn the AC on. It occurs mostly around 2.7k to 3k rpm
(3) On a timing light, the vacuum advance operated in a jump but not in a smooth way.
My guess is, due to increased tension of actuator plus reduced vacuum from ported line at WOT, the vacuum advance disengages early before the mechanical advance can start playing its part.
The question remains, how to test it. A person with detailed experience with timing curvez can easily verify this using a timing light. Normal road-side mechanics cant do it. They would rather throw in a new distributor without tracing the cause.