Eliminate the easy stuff as possibilities. Your timing, vacuum leaks, harmonic balance slip.
Also your idle screws only help with good idle not with the transition circuit. You can verify a good pump shot looking down the carb and twisting the throttle open. It wouldn't hurt to rebuild the carburetor and check that you don't have any plunged ports. Might be worth while to just pull the carb top off to see if you have a bunch of crap in the bowl.
However from what you describe with the vacuum gauge I'm with Al. Pick up a compression gauge or rent one at Autozone and see what you get the motor should be up to temp and do the compression check with the carb wide open.
I would also suggest a leak down test. Harbor freight has a cheap one that will do the job. Check out some youtube video on how to use it if needed.
