Any air/steam leakage any where will reduce low speed performance. Put gaskets on steam chest covers, between steam chest and cylinder and cylinder covers. As mentioned, a good piston/cylinder fit and gas tight seals on the valve stem and piston rod.
Put your throttle valve as near to the air/steam inlet to the steam chest as possible. If running on air with a regulator, use a large diameter hose from the regulator to a needle or ball valve near the steam chest. The larger hose will provide a reserve of higher pressure air to supply the throttle valve with. Set the regulator pressure at least twice as high as necessary to run the engine with the throttle valve wide open.
Looking at Rick's video, note the regulating valve near the steam chest with the regulator back a ways with a large diameter hose.
We all talk about how low a pressure an engine will run on, but this is not the way to get the lowest speed.
If you look at steam locomotive plumbing you will often find the throttle valve in the front of the locomotive boiler in the smoke box near the cylinders and operated by a long rod from the engineers control. Sometimes theis rod runs through the boiler. This also kept the steam hot from the throttle valve to the cylinders.
Gail in NM,USA
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