And some more pics / video because I want you all to hear and see what this is doing.
I got it fired up and running again, but this time I changed the setting on the tach. Sure enough, at half the measured RPMs from before this thing makes a little more sense:
It seems like it doesn't want to idle correctly. It won't go below about 2.7k rpms, even at low throttle. The thing is still running a little choppy too. It seems like I can smooth it out by turning the low idle fuel adjusting needle to a really rich position; not sure if that is working as intended. That was the fix from another video I saw. It also gets a little smoother as the engine warms up.
So... here is a blown out diagram of the carb from the service manual:
I have tried backing out the low idle speed adjusting screw and spring with the mower running and no matter how far I back the screw out, it doesn't seem to make a difference in the low-throttle idle speed. In contrast, if I tighten idle speed adjusting screw then I can get the engine to run faster at low throttle. This is in accordance with the idle speed adjustment instructions from the service manual:
So is the main jet messed up or what's the deal? This is the first carb I've ever worked on so I don't have a lot of perspective. It seems like this thing can only be adjusted for low throttle idle and I have that turned up so high that the mower is just running on "low idle" all the time. Does that seem at all likely?