1st Gear Hesitation

[jmn]

Hi - Wasn't sure where to post this since I don't know what's causing the problem. I have a 1962 CB72. Idles fine, and at first after starting, revs smoothly with the throttle into 1st gear. But soon after it warms up, when I'm in idling, then shift into 1st, when I twist the throttle there is a lag, and I have to really twist the throttle to get it to power up and pull into 1st gear. Once it's moving, this doesn't seem to happen with higher gears. Anyone have any thoughts???

Thanks!
jmn