Bonjour Derek,
For information, 1 bar = 14,5038 PSI
Personnally, I don't like starting using a steam engine on air, even if you try to lubricate the cylinders with a light oil.
I like starting at about 0,5 bar (+/- 10 PSI) with a lot of oil everywhere, especially steam oil for the cyllnders and I let my machine run on my table for 1/2 hour adding regularly oil on the moving parts.
The oil should become grey, normal, first frictions.
After,I like using my machines at 2 bar (30 PSI) as wet steam is also auto lubricating.
Little example :
https://www.youtube.com/watch?v=kFyBAtKFvXYBesides this, this 7,39 cm3 deserves a good boiler, which one do your have ?
As steam machines have a good torque and do not need to run quickly, I like coupling my 7-8 cm3 to 4 blades 80 mm ( 3") propellers