[Christ]

Use a lighter spring on the clutch/drum assembly... that will let you go slower.

You can change the "stall speed" as it were, of the clutch assembly by messing with the spring and counter weights... lighten the weights, or stiffen the springs, and you have a "higher stall"... lower the spring rate, or make the counter weights heavier, and you have a "lower stall".

Something I used to do with go-karts after tuning them to run on Kerosene (less power, better efficiency).