I have this one ready to put back together but don't have to much time to play (no comments Pete if your reading this) but when I get caught up I will install it on a runner and see if I can keep the fuel up but the smoke down.
Chris you know I respect your thoughts, input and knowledge. The thing with loosing the buttons is weird. My thoughts were if the HH or bottom of the HH with the button is always in contack with the tappet roller and the roller is always in contack with the cam then the button should stay in place. My thought is for some reason the plundger sticks, maybe just for a millisecond but that would be all that is needed to get a gap between the tappet. Then the tappet comes back up on the next pump stroke and smashes the button causeing the button retainer to come off and of course then the trouble begins. My thought on you thought is if the spring retainer is still on the bottom of the HH and in place when the plunger is compressed up to max it would stop the plunger from going any farther as it would be a solid. I will grab a HH today if I go to the property and take a pic of what im thinking.