Morris, those air start engines would need all of 250 psi or 17.2 bar of pressure to start and a lot of it. Also, in the rim of the flywheel there is a series of holes into which you put a bar to use to rotate the engine until just past TDC before opening the air tank valve to give it a shot of air to start it rotating., but you probably knew that.
There used to be a ferry on the Hawkesbury River doing the Postal run each day. It had a single cylinder ATLAS air start engine. It had a large capacity air compressor in the engine room and it had to work hard to get the pressure up in the large volume air tank before each start. Once it was fired up in the morning, it was left to just idle between jobs.