Hi Tom,
I am no expert on engines that use atmospheric pressure to open the inlet valve. I too would be very interested to hear from someone with experience here.
Regarding the exhaust valve staying open after TDC into the inlet stroke, this is a design for higher revving engines where inlet and exhaust valves overlap a little to help start the gas flowing as early as possible. IMHO this is not applicable to these engines for 2 reasons, 1. the engine speed is very low and 2. if the exhaust valve remains open into the inlet stroke then part of the inlet stroke is wasted as reduced pressure cannot form in the cylinder to open the inlet valve if the exhaust valve is still open. However if the exhaust valve closes too early and the piston is still rising to TDC then again positive pressure will be in the cylinder at the begining of the inlet stroke. Therefore i would suggest the exhaust valve closes at TDC.
The exhaust valve duration is strongly effected by the valve clearence, we are taught that long duration is good for power, but again only in high revving engines. The exhaust valve duration on these engines seems quite long with a 'normal' valve clearence setting which results in the exhaust valve starting to open about 1/2 of the way down the power stroke when set to close at TDC before the inlet stroke, so maybe more power could be gained by increasing the valve operating clearence to reduce duration and setting the exhaust valve to open later toward the end of the power stroke but still close at TDC before the inlet stroke?
I'm sure Jin has looked at this and chosen wisely, but these little engines are great to experiment with :-)
Kind regards
Ade