What Digger is saying, is when the vacuum diaphragm leaks,... the symptoms are most apparent at high vacuum conditions... which is at idle and low speed steady cruise...
At Wide open throttle, vacuum is close to zero, and so, very little gas is sucked down the vacuum line directly into the intake... so no symptom at full throttle...
Let off the gas,... vacuum goes way high and gas is drawn down the vacuum tube, and the bike stumbles rich...
Full throttle,... no symptoms
Low speed and idle,... stumbling, dying, backfires...