Thanks for your suggestion.
I tried adjusting the height between the reader and flywheel while the wheels were moving and at startup. It didn't change anything. I tried blocking the sensor with a piece of white paper to see what would happen. The motor when faster and faster but the chuff rate did not change. As soon as I removed the paper, the motor slowed down to scale speed and the chuffing adjusted, but still chuff 23 and not chuff 234 as it should be.