Just had a chance to check out the mechanical advance and it appears to be functional. I also put the mark on the damper to see if it's moving. And, I ran it again, and I can pull about 4700 rpm's before the secondaries kick in, where the problem manifests itself. I tried putting my hand over the secondary venturies to choke it down a bit, but I couldn't get a good cover over them.
No, I didn't change the fuel, but I've run into bad fuel in the past, and usually the problem is the engine simply won't start if the fuel's taken on enough moisture. Not saying this couldn't be the issue. There was about a third of a tank in it, and I filled it up, so I might've diluted the problem.
It's a mechanical fuel pump, but I'm not sure of the pressure. I'll have to check that.
This carb is the type that you have to adjust float level by bending the arm on the float bowl; there is no external adjustment. I turned both bowls upside down and made sure the floats were level with the bowl when that was held level with the ground.
Thanks again for the input, guys. Any other thoughts are welcomed. Not sure if I can find a carb swap just to see if the metering is way off.