My C6 shifts a bit late from first to second gear ever since I bought it and I figured out that it is probably because my VRV is out of adjustment. I looked into how to adjust but couldn't find a detailed article. Where do I hook up the vacuum gauge and where is the screw to adjust it? Thanks

 

I had the same problem w/my 86, 6.9L, C6.
There is a vacuum valve on the left side of the IP attached to the throttle shaft. It has 2 vacuum lines coming out the top, and 2 screws in slotted holes. Loosen the screws and rotate CW (top towards the cab) to lower the shift point. Go slow, a little movement will made a big difference, just keep adjusting until you get it where you like it.

Steve

86 F250, std cab XLT, 6.9L, C6, 3.55 ltd slip diff