Glad you finally found it. :thumbsup:
Wasn't that suggested as a cause about 30 posts ago? :poke:
This is certainly an odd (but a good) example of the rule that advance should always start above idle speed... (unless locked-out of course).
Yes, post #27, took distributor apart to check but of course hard to verify a weak spring by looking at it.. and I didn't want to start revving motor until I put everything back together and manually turned over the engine with breaker bar while watching rotor to ensure no slop or play in cam timing. Once that was one and I had confidence in the timing gear/chain, I fired it up with the light and got to work checking the curve.