I built my 4L60E. It works great but when I start the car it jerks back a second. Still on park. I've had these jerk forward before when the fwd clutches were on the tight clearance side.
Can't think of why this would jerk rearwards in park mainly on cold start.
Any ideas?

 

The Low Reverse Clutch comes on in Park.

 

Any idea why it pulls back? Maybe low/rev tighter than fwd?
Seems normal for it to jump forward to me. Never had one jump back.

 

Tight Reverse Input clearance? Shots in the dark of coarse.