Transmission specialists know that "tranny trouble" frequently follows carb work.
There are two adjustments that must be made. First, the kickdown band on the outside of the transmission must be properly adjusted to a torque spec and certain number of backed off turns that is described in the FSM.
Second, the linkage itself must be adjusted. You would think thatt's simple enough, but the FSM describes about a dozen points that must be lubricated and/or adjusted in sequence starting with the accelerator pedal.
When you have everything in the ballpark and there is no intereference or sticking in the linkage, lengthening the adjustable rod at the carburetor results in later (=higher RPM) upshifts -- your Imperial will behave like it's a Roadrunner if the rod is too long. If you're in 3rd before you get across the street, the rod is too short.
Adjust the rod until you like the shift feel during normal city driving. Then take it on the freeway and see how you like the shift feel on WOT acceleration. Only a small degree of change will have a noticeable impact on shifting, so make your corrections carefully.
The technical explanation is that shift points are determined by a balance among transmission fluid pump pressure (constant), governor pressure (determined by vehicle speed) and throttle pressure (determined by foot pressure on the accelerator pedal).
Assuming that your pump pressure is good and the linkage is properly adjusted, if you are still having a problem then make sure the governor is not sticking.
Once you have adjusted the rod length to your style of driving, experiment with forcing a kickdown and continue adjusting until you are Imperially satisfied.
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