I just encountered the same issue on my M3W. It has been idling a little fast recently, and on my last drive it was hard to keep running after starting, idling with strange chuffing noises then stalling, although running great once under power.
Thanks to AlexKnoll, I remembered this thread, pulled my S&S Stealth air filter off to take a look, and when I rotated the throttle I could see the bottom of the O-ring had popped loose inside the throttle body. It turned out that the head of the bottom retaining bolt for the throttle body had sheared off, so the only thing holding it on was the top bolt. I guess it was either a defective bolt, or when I mounted the air filter I must have put it under enough stress to shear it off. I'll be careful when I re-mount the filter to make sure it is not stressed. Others with this filter might want to look out for this issue.
Ordered a replacement bolt and spring washer today, and am hoping that this will resolve the problem. I'd have been paying for a tow truck and a mechanic without the accumulated wisdom here. Thank you!
Where did the sheared off head of bolt go?
