I have the same short shifting problem as the post by JRHPAH with my TH400.
I have kinda fixed it by using a Corvette Gov., it will drive out normal in first most of the time.
Sometimes at low speed 5-10 mph it will shift into 2nd and then want to wind out alot before it shifts int 2nd.
I also have the modulator screwed in alot as I have a low vacuum motor 7" @ idle.
The tranny will also downshift into 1st. from 2nd at 40-45 mph under med. throttle?
Could this be from a worn or bad governor bore?
T.C.

I don't think that is the problem. B&M makes a governor recalibration kit that might help, part #BMM-20248. Do you have a shift kit in the trans? If not, that will help also.

You need to tweak the governor to get shift points where you want them. As to governor bore, you can make a go/no-go tool to check the bore size.

Thanks for the replies:
I actually have it shifting pretty good, if I could get rid of the downshift into 1st. at med.hard throddle problem.
Kinda scary going into 1st at 45-50 mph, goes rite onto the rev. limiter.
I just thought maybe the gov./gov. bore was causing that?
T.C.

If the governor bore is bad it would be very worn, scored, or cut so would the governor. The governor could also be loose at the base.
The first thing I would try is a low vacuum modulator.

Thanks for the reply, If ya mean the modulator with the large can, it looked like it was off something from the 1950's ?
I all ready did, Didn't make any difference?
IF that was the wrong part, please let me know and if ya have a part number that would be great.
T.C.