One last attempt to see if a steam test did more than an air test. While I am not convinced the valve timings are set perfectly, it's easy to experiment if you are running on rollers - just kill the fire, fiddle with it, and light up again.
One silly issue with this loco is the gas jet, which is attached to the tank. Nice idea in theory, but in practice it means that you can only fill the tank once before you have to stop and let it cool down. What happens is that it gets hot, the gas pressure inside rises, and the gas in the can can't overcome the internal pressure. (A few Accucraft locos have this same problem - it's endemic to gas tanks close to the hot boiler.)
A solution is to cool down the tank. As I am running on a drop tray (borrowed from the toaster oven,) I dropped 3 ice cubes on the tank and jet. It worked once the ice melted, which happened quite quickly.
Anyway, several adjustments later, we were close but no cigar, as the saying goes. The wheels attempt to rotate but stop every revolution - interestingly at the same point as they did a few tests ago. Here's the starboard (RH looking fwd) side after it stops, with the piston fully back. At this point this cylinder isn't doing anything - it's up to the other side to rotate the wheel enough so that the steam can get behind the piston and push it.
And here's the port side. It is 90 degrees different (correct) but it stopped. At this point the piston should be pushed by the steam so the wheels continue to rotate. Even more curious, there was plenty of push until the rotation got to this point - possibly done by the other side cylinder.
I am still of the opinion that the other side is doing all the work and this side is not doing much, although it does let the wheels rotate when about 180 degrees from this, as the other side piston reverses direction. Then after the other side pushes everything vigorously for 1/2 turn, it all stops as if there is a spanner in the works. And it stops at the same place.
While doing this testing, the steam pressure drops to 1 bar, about 15 psi. That isn't enough to drive any steam loco in my experience, so that should be my focus next. I believe the rotary valve needs lapping in (having the rotary faces smooth so they don't loose so much steam.)