Another thing you will probably notice if your engine is new, as it runs for an hour or two, it will free up/get run in, and use less and less air to maintain the same rpm, an engine such as the Stuart Turner S9 is designed to run at 400rpm, and if the govenor is working properly, it will hold that speed fairly closely, on the engine I rebuilt, on a pressure range from 20psi to 60psi.
The S9 got nicked, and we have not found it yet B***ER.
Ian S C