There are 2 screws in the handle that are used to adjust the locking tension.
The manual I have refers to this but gives misleading info as to their
location, which leads me to believe that there are differences in the models.
One screw (on the side) locks the adjusting screw (under the front).

I have found that in order to keep the mechanism from sliding down to the
lowest speed, the locking tension must be so tight that it requires great
pressure to use. So I have devised an extension arm that slips over to handle
to provide more leverage.

Perhaps there have been changes in different models and years. For the two R/D
12" HD lathes I had the levers were clamped by overcenter. That is you pulled
the lever away from the lathe, then turned the lever to adjust the rpsm, then
pushed the lever back overcenter to lock it in place. Often, students would
attempt a 'gorilla' move and try to turn the lever without unlocking it first.

Dan