Help diagnose a sputtering bike?

[UltraLife]

I have a 2006 Bajaj Chetak which has served me well for a long time. I rarely start it during the rainy Seattle winters, but drive it a lot in the spring and summer. I went to start it up the other day and it starts and idles fine, but when I turn the throttle, it sputters and has very little power. I made a video so you can see what happens.
Can someone help diagnose this?
Big thanks in advance!
Adam