Ralph is correct. All of my Walbro choke levers operate such that I can cearly feel a strong "click" when the ball falls onto the detent. I can't imagine an engine vibrate enough to make it jump out and stay off of the groove.

Depending on the orientation you have on the choke lever, you can hook up a pushrod with clevises on both ends. Fix one end to the choke lever, clip the other end onto a fixed point on your fuselage (you can screw a control horn to the side of the fuse for this). Now, you can unhook the clevis off the fuse and work the choke with the pushrod. Then when your engine has warmed up sufficiently, clip it back on the control horn, so it will never come loose in flight. You can even set up another control horn to hold the choke closed.