1984 f150 XLT 4x4 351w c6 all new built or rebuilt. Transmission shifts great through driving range however when it's cold it doesn't want to shift into drive, and sometimes not reverse. All shift points are good while driving, just that when cold, it doesn't like to go into drive. I've been reading about he modulator and front clutch seals but it was just rebuilt so the seals and clutch pack are new. From what I've read the modulator controls the shift points, making them earlier or later depending on the depth of adjustment. What is the deciding factor of the initial shift into drive or into reverse? It's also been mentioned that the electric choke may be playiby a part in the issue as in increasing the choke while warming and thus changing the vacuum.

 

Pull the vacuum line off, there is a tiny straight screw in there. Turn it out 1 full turn.
Try that for start. You may have to tinker.
don't go more than 4 turns either way
Write down what you do, every time you do something, so you can return it back to stock

 

If you pull the vacuum line off, and transmission fluid comes out, the mod valve is bad