C4 shift adjustment

I have a 66 with a very mild 289. I have about 19'' of vacumm at idle. The C4 was rebuilt about a year ago and has always shifted too early. I almost always run it where it starts in 2 and shifts to three. It will shift as early as 15 mph if i am taking it easy. When I use the 1-2-3 selection it's even worse, it almost immediately shifts to 2nd and shortly after to 3rd. I have tried adjusting the vacumm modulator and it seems to have little effect. Will the band adjustments raise the shift points and which one does what? Thanks for any input.

Thanks for the replies. One other thing, when coming to a stop, I have to almost come to a complete stop before it shifts back down to second. Any ideas? I feel like it should shift down earlier when decelerating. Thanks for all the input.