John,
Having built one of these also I have to agree with Bogs - this particular design is extremely fickle! Not wishing to put you off, I think it's a good idea and I have seen some flame lickers which have such a valve to release pressure as the piston nears TDC, but those were engines that had the valve driven by a cam.
In this one, as you rightly say it's opened partially by pressure on this one but that pressure is a tiny tiny pressure, due to the necessary low friction of the system it opens as soon as the pressure rises above atmospheric - remember there is no spring holding the valve shut, and there can't be one otherwise it won't be open long enough to draw in the hot gas.
I ran mine again the other day for the first time in ages and every time I am surprised how critical flame position and steadiness is. To give you an idea, 1mm in flame position can mean the difference between doing a few hundred RPM and not running at all, somebody moving in the room or opening a door can disturb the flame enough to stop the engine - they are a nightmare to get running but a joy to watch once they are!
Nick