I have driven my 72 CS a few times and never encountered this issue that I did today. I positioned it in 'DRIVE' and upon accelerating it sounded as if the RPMS were increasing yet the speed was not. I flushed the tranny, replaced the gasket & filter about 1 month ago, thoughts? I tried shifting through the gears and still encountered the same issue.

 

Are you sure the tranny has enough fluid? If it worked fine before the flush chances are you just need to top it up?

 

Please note, I moved your post to the correct category.

I would check the hose to the modulator and/ or the modulator itself. Item #2 would be to check the detent cable between the gas pedal and the transmission to see if the trans is hung up in passing gear.