Hey Gil
The mini-Robinson is in fact vented. If you will look at the tripod foot opposite the flywheel you will see that there is a very small vent hole right at the toe of that foot. The foot actually partially blocks the hole on my unit, but I have drilled it out such that I can use my blunt needle hypodermic to put alcohol (about 2ml) in the tank without unscrewing it. It will occasionally fire a long blue flame out the vent hole however, so watch out for that!
I'm way impressed with just how well these tiny engines run.
On another note, I do run mine on a ribbed rubber mat so that it behaves itself while running and doesn't dance around all over the place!