The story so far goes like this. TH700R4in a 86 IROC, Fault was it would not stay in 4th gear if you pressed the gas pedal, hard or lightly. Eventually lost 3rd gear, so bought a rebuild kit and rebuilt the gearbox.
Box installed and still the same problem. Fitted a new TV cable and adjusted it several ways as advised by manuals and suggestions received through this website. Still the same problem.
The car selects 1st, 2nd and 3rd between 1,750 and 2,200rpm and is very smooth as it should be, but the rpm rises to about 3,200 before it goes into 4th, and when it does go into 4th if you touch the gas pedal even a little bit, it drops back into 3rd !!!!!!
I really need some sensible suggestions as the summers a comin in Scotland and it dont last too long !!!!
HELP Alan.......

 

I have one more suggestion now that I know it is revving high before shifting. Is it possible that you could have cut the o-ring on the servo? I know the first time I rebuilt one I had trouble because I cut it myself. If you have any easy way to get under the car then check this first. It is really kind of difficult to mechanic over the computer, but I will help you figure it out if I can. Good luck!

 

Where in Scotland are you?

 

Took out the servo to check the seal and it was ok.
Still baffled......

 

I would install the Trans-Go Shift kit (green & white box) & would get another TV plunger sleeve (from a later valve body 1988-1992, such as an IROC, Z-28) with the 4-3 part throttle holes further back so as to get a later (harder to get) 4-3 downshift. You could eliminate the 4-3 downshift altogether, by blocking the 4-3 part throttle downshift holes if desired.

Note: In the near future I will have the pictures of how to do this on my site, as well as other modifications.

 

When you get the instructions on your site for blocking off the 4-3 plunger holes I`ll give that a go.