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Hey gamg, need some quick guidance. I just finished the build up on my 65 which included the addition of a 390 and a C6 that I refreshed. On our maiden voyage, everything appears to be in order but for one thing. I have only been running in the "D" position, granted this has been around the neighborhood but the 1-2-3 shift seems to all occur rather quickly. They are not hard or sticking and I have not really put any peddle behind it off the line yet. A Transgo kit was installed also.
Your vacuum modulator should have a adjustment inside it that takes a small allen wrench through the vacuum line plug in. Turning it in will slow down the shifts. Try 1 turn at a time until you get it where it should be.
Quite often when shift kits are installed the minimum shift points are a tad too minimum.
If turning the screw in the modulator doesn't help much you can try lengthening the pin.
I use a metal coat hanger and it works well.
Measure the old one and make one .060 longer.
Round the ends and give it a try.