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The vacume modulator is the key. Not sure they are all adjustable. During the 70s for emissions reasons, some of the vacume diaphrams had two ports, one to offset the normal vacume pull undercertain conditions.
I had a 73 Gran Torino with a 351 Cleveland and a C6 that had this double vacume hose on the diaphram. The car would shift VERY early untill I pulled the "extra" vacume hose.
Ed
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