Hooooooray I found the air leak!
Second build is now roadworthy, after I lucked into solving an air leak. The RPMs would race away, so I did the carb cleaner test and came up with nothing. I was worried I'd have to replace crank seals, but remembered that I still had another carb o-ring from SBP. I vaguely remembered that it would fit in the carb top, so I gave it a try.
Perfect throttle now, probably better than my other bike. I am so ready for the weather to break.