Sorry about taking so long to respond. I did resolve the issue by replacing the carb again, but I'm not sure exactly what was wrong with the one that was on the engine. It definitely wasn't gummed up, because it behaved that way right after installation. I believe it was a factory assembly, but I seem to recall having to migrate the jets from my factory original carb. I suspect I forgot a part or otherwise messed up when moving them.
With the carb that was acting up, I tried to back out the slotted plug from the underside (the bowl area), and it was seized into the aluminum threads. After mangling it with my screwdriver, I ordered another factory carb that came with the jets preinstalled.
Before mounting it to the engine, I compared that new replacement to the one that was having issues. They were 100% identical except, when viewed from below, the slotted brass plug on my previous one was recessed about 1/4" higher up in the carburetor than the new one.
So either I significantly over-drove it when installing the jets or I missed a part, such as an orifice disc, when assembling. I'll never know for sure, though, because there was no way I'd get that assembly apart without cutting it.
The new-new factory carb has worked great for a couple seasons now with only a minor adjustment to the idle mixture.
