Rich Grise wrote:

Centrifugal clutch, a la:

http://www.gokartsupply.com/clutches.htm


We thought of that, and it might be a solution. Apart of karts, maybe
some from garden tools, chainsaws etc could be source of cheap ones.
For them to work, I think pull-off springs would have to be changed for
very light ones, or maybe no springs at all.