It's probably just misfiring - I don't have a good ear for this car or American V8s in general yet (used to aircooled 4cylinders and Italian stuff...) but I don't hear anything
that sounds like pinging.
By "unplugged" I mean I remove the vacuum line from the carb and cap off the carb.
I tried 5 and 10* as well, before trying 0*. Both had the same issue, although slightly lesser with less initial advance. I figure if I have to go to ATDC just to compensate for the vacuum advance adding too much timing,
then something is not right.
Rotor phasing actually sounds like a pretty likely culprit here, since it does seem to be a misfire more than a pre-detonation scenario. But if that's the case, how do I fix it? Can I? The rotor itself isn't adjustable on this distributor AFAIK.
Been a while since I explored the phasing issues, but there some small things that can be done depending on the cause. For example the cap alignment can be shifted a little, or the advance itself. I've done the check using a cheap summit cap but haven't had problem.
Might begin with checking the spark plugs. See what they look like. Carbon fouled? Clean?
Looking inside the combustion chamber like that can't hurt and could provide some insight.
Thinking about this the Shrinker way, what is going on in the chamber that the engine revs fine with no load. (I'm assuming that since you've measured timing from 700 to over 2500 rpm). But with any load there is some issue lighting or burning.
We can go further. At no point has there been a backfire.
So lets think about what is happening in the combustion chamber under load versus free reving.
Is more resistance from the piston load putting the spark out?
Maybe its not even getting that far? Could the conditions at the gap under higher compression be increasing the voltage needed to make the jump? The spark plugs may show evidence of these situations
If so then why isn't spark energy being developed or is getting lost. Rustyratrod has mentioned dwell, and anything related to that (condensor, too much resistance in coil feed, etc.). PRH's mentioned phasing, but also anything related to the spark having extrat difficulty making the jump under the cap.
Anyway some stuff to think about when looking at the spark plugs.
American v-8 or air cooled 4 some things are going to work pretty much the same way. Ever drive something or run a small engine where you forgot to reduce the choke? Same type of experience you'll have with this engine if it was too rich.
