I'm wondering if this engine is like the .61SF, the carb specifically, in make up, not size.
If it is, there is a cam that goes on the side of the carb, that adjusts the mixture(slight adjustment at best), anyway, the cam actually moves the spraybar within the throat of carb to attain optimum air/fuel flow.
That cam will only work correctly when installed ONE WAY. However, there are 4 ways it can be installed(if I'm thinking correctly here). I know the 61SF has the cam and that's how it works, and a person can go nuts trying to figure out why it won't work(don't ask how I know).
You said you rebuilt carb, that's what keyed me to this, and the symptoms you describe are exactly how it would react if it is a cam situation.
If there is a cam on that carb, make doubly sure it's letting the spray-bar aim into the throat of carb, yes if it's there, there is a way it will work, if there's no cam, follow all other advice you see here in this thread. Good Luck
Dale
PS If carb is similar to what I mentioned, there will only be about 1/4 of a turn total adjustment for the mixture, that's about what the .61 has, and once you get the hang of these carbs, they are not bad at all.