Find the u shaped part on the frame where the belts run past that seems to serve no other purpose than to keep the belts from wobbling. It is just below the top sheave but I can't remember which side. Just under that, using you middle finger, push directly in on the belt. Disengaged, you should be able to push your middle finger in so that the second knuckle sits under the u shaped belt guide.
Yep, this is part of the pre-flight checks. The belt tension appears fine when I do this, on both the R22 whose rotors start immediately I switch on, and the one whose rotors take....maybe 10 seconds, not sure.
So it appears that belt tension isn't the problem. The micro-switch explanation seems to make most sense so far.
I'm surprised that there seems to be no concensus; I thought I must be the only R22 pilot in the world who didn't know about this. Obviously not.
Any more suggestions. Lu, where are you when we need you?