Is you're kick down linkage adjusted properly? Is it even hooked up? Proper adjustment is with the caruretor at wide open throttle the lever on the transmission should be all the way to the rear. Essentually both throttle lever and tranmission pressure lever should hit the end of thier travel at the same time.

Here is the tricky part, with the throttle fully closed, the transmission lever should still be back about half way, or maybe just a little less.

If you have swapped to an aftermarket carb this can be very difficult to achieve without switching to a fully adjustable cable system like lokar.