Something is wrong with the spark advance.
Opening the throttle should move a linkage with control over the ignition timing.
I speculate that this is not happening and while extra fuel air mix is being introduced, the timing is not right for it to run correctly.
Observe the frame that holds the ignition components under the flywheel.
There should be a linkage from the carbs up to it.
That linkage should rotate the frame as you advance the throttle.
Don't expect a linear relationship between the two.
It may start moving only a small amount, then a large amount and finally have no more movement with some increased throttle left to be had.
A link between the carbs and the ignition may be unclipped. Look closely!
P.S. no 6 pack needed, regular Oreo cookies will be just fine!
