Anyone well-versed on A-904 tranny?

yeah man! you heard the guy right.
That little lever might as well be called a throttle position sensor. It tells the trans when to shift. I check the adjustment whenever I do a tune up, Also if you change/add a carb spacer you'll have to adjust it again.
I was a little surprised at how a small movement of the lever made a HUGE difference in how the trans felt. Too far forward and you get long gears and quick jutting shifts, not to mention the instant downshift when you even think about hitting the gas.
Too far back and you get early soft shifts, not enough RPM to keep the thing moving and run the risk of burning out the bands.
The key is to set it up for the way you drive. then put a mark on the linkage so it can be put back that way after you're done tinkering. It is zippy in the forward position but too twitchy on wet roads.

where do you have the initial timing set? there might be more fun to find :)