A friend within the past few days installed a B&M transpak in a Pontiac Firebird with a 455 and th-400. Went by the directions i think! Anyway the car is now shifting itself around 6500rpm's. Engine is stock with headers and exhaust. What would cause it to shift like this? It's in drive when it's doing this. If you shift it manually it will hold it until the appropriate rpm.

thanks for any help,
Rusty

 

Make sure that the vacuum line is still connected to the modulator. The other cause could be a missed check ball. It sounds as if it is shifting on the governor only.