Is my carburetor missing the center adjusting screw?
Not all AFBs have the center adjusting screw.
I’ve tried two different carburetors and I still can’t get it to idle right…
Well the fact that it does the same thing with 2 different carbs tells me to look at electrical 1st. Compression test might reveal 1 or more low cylinders which will cause an idle problem for sure.
I wouldn’t discount a vacuum leak that hasn’t been found yet.
The center screw not making a diff is worrisome. That smells of a vacuum leak to me.
Double check the mounting gasket to be sure it isn’t leaving some port exposed causing a leak.
Spray carb cleaner around all vacuum sources and mounting gasket. A change in rpm tells you that you found the problem.