Rebuilt turbo400 shifts through all 3 gears by the time I get to 20 mph. The vacuum modulator on the tranny is connected to manifold vacuum since the carb does not have an extra port for it. Is this the problem or maybe the modulator. Not sure where to start.

Manifold vacuum is correct for the vacuum modulator on your T400. Have you tried adjusting the modulator?

Alan

Are you talking about turning the screw inside the metal tube? I've heard that doing this only makes the shift firmer not really solving the problem. Have you ever done this and what was your results.

Adjusting the modulator is the correct procedure for this problem. It will delay the shift - but firmer shifts are a result of a delayed shift. The shifts will not get "harsh". I would start by turning the modulator in (clockwise) two full turns. I have always done this to get the shift delay I desire.

To answer your followup question, Stephen said exactly what I would have. Let us know how it works, as the validation becomes a valuable future reference.

Ive also read that there are some modifications that can be done to the governor that will delay shifts. The modulator adjustments will change part throttle shifts but on slow rpm building shifts-it wont change these much or if any. I believe if material is removed from governor weights,this causes delay.Think either Jegs or Summit sells kits with different springs/weights(like a dist recurve kit)