Re: Eratic timing marks at idle with timing light.
David, my bad! I just went back and read your post #7.
I guess between the two of us, we have doubled up enough info here to cover both the East and West coasts! LOL
As for the TA or TAT.........
Yes, and I don't doubt for one minute that you have experience in this area. You sound very knowledgeable at/in this area!
However, I couldn't help but catch your statement in post # 11!
IMO, that could cause an inexperienced person to think that BASE is the only and main advance spec to be concerned with, and that setting Ignition timing
at the higher RPM (TA or TAT) could cause poor performance from an incorrect BASE advance.
TAT on a marine gasser is very important, and IMO, much more so than BASE, if we're splitting hairs between the two!
That's all I was trying to convey here, David......... no offense intended!