underburning on an optimus 111c

It seems so me that what is happening on the 111c is that when the stove is turned down to simmer, the fuel/air ratio falls to an explosive level, there is a pop and the kero/air mixture ignites under the cap, where it is confined by the outer cap.

Possibly this could be avoided using the slide valve meant for reducing the air flow when burning alcohol.

The effect might depend on the chimney effect of the riser tube. When the stove has been on high, the tube might be hot enough to entrain additional air - I don't know how significant the chimney effect is on a small tube.