You need to check all the basics (spark/fuel/compression/timing)
You have not mentioned compression, if it is low then the next step is checking the valve lash. It’s all in the FSM which you can download from here.
Those are all on my list. I’m just baffled that for months while finishing things up I’ve been starting it and letting it run a minute or two 3-4 times a week and have had no issues with snappy throttle or what I have now. Then I think it’s ready to run, put all the body panels back, and now it’s not happy.
I did a compression test last summer and got 115, 120, 120, and 135 from 1 to 4.
