OK , Thanks guys .
I thought it was a 'cruise control' type , but wasn't positive . I couldn't find any type of spring in the parts book .
I'd thought about just going with a motorcycle / minibike twist control like Reese has , but I like the idea of concealed cables to preserve the clean look .
Anyway , I developed a push / pull type control where the right grip slides up and down ( on and off ) the handlebar instead of around the bar .
Here are the parts :
The black tubes ( grips ) are pieces of aluminum tubing I salvaged from a broken collapsible canopy leg .
The brown pieces and the grip end plugs are hard wood salvaged from an old broom handle . These are held in the tube(s) by a few brass brads which are removable if service is needed . The piece on the cable is a centering plug that keeps the cable from too much bend while in service . The honed out piece fits on the tapered side of the cable end knob and the flat side of the knob sits against the grip end plug .
Here is the cable and control together :
And here they are on the handlebar :
This pic shows the throttle closed :
This pic shows the throttle wide open :
From closed to wide open position is a little more than an inch .
The control and cable end are held in place by a set screw against the cable where it enters / exits the handlebar , close to the handlebar mount . No springs involved .
p.s. : I'm thinking of covering the tubes ( grips ) with thin foam like is used on bicycle handlebar covering .