To check the mechanical (or any 'n all advancing going on) would'nt it be easiest if you used a timing light while revving the engine..? Locate & put a big blob of white on the pully timing mark so it's easy to see... I used my wife's fingernail polish pen with white color...

Actually, I don't know if my timing idea will work on a fuel injected ECU engine, I assume your motor has a distributor, maybe not..? I would just remove the dist cap and try moving the base plate parts that move when the mechanical advance is activated... Could be things are stuck or in need of cleaning & some lube... Try not removing the distributor unless that's the only way to fix it, too much work in my opinion... Sometimes we break more than we fix, I do, so it often pays to do only what's needed & no more... I have a 50/50 success rate...

Hoping for your Success
George