Hey everyone! I got a '75 Cat XR7 with a stock 351W to a C4. Over the last year I had to drive this thing on the Interstate for long miles at a time. I noticed especially when the car was cold it refused to make the jump to lightspeed (3rd gear) unless the engine exceeded 4700. 1-2 shifts still worked great, making the jump at 4500 religiously. 2-3 was really slow.
I figured it was the vacuum modulator, but now after I changed it, adjusted it, and re-added the fluid to the correct level, 1-2 now shifts at 2200 in drive and it wont go into 3rd for about 10 minutes of normal driving. When it shifts, the engine revs and waits a few seconds before the transmission slowly catches and sluggishly slides into 3rd, instead of the quick pop shift it used to do. Adjusting the modulator does nothing anymore either!