Hi Doug. I'll be interested to know how you set the valve port sizes and slide valve dimensions. As you'll be aware, lap on valves is quite a critical bit of the design of the engine. I am making a steam pump and the valve timing is adjustable, to compensate for (MY) inaccuracies of machining. But you don't appear to have any adjustment if a port or valve slide is a few thou "off"? I don't want to suggest your machining is as bad as mine, but I have only built engines with some adjustment in the valve train, to tune to balance (centre) the valve when I get to the running stage. That way I do get very smooth and slow running engines, without an odd "kick" or stall at the end of one stroke...
Also, what material are you using in the glands? I always used graphited string, and have put 1 or 2 turns of PTFE tape on the shaft first, but my latest uses silicon O-rings... which I am sure will seal, but hope they will slide smoothly, and not bind. I may be tempted to apply a layer of PTFE tape to reduce friction for when I tighten the glands "just enough" to stop steam leaks.
It looks really good anyway! - Looking forward to seeing it run and make some power!