Have you pressed the reset button on the Tiny tach? I have had a freeze on my Tiny tach and a reset fixed it. When they freeze up I suspect the tach is not receiving a clean signal. I have this issue with my point style Y80 but not the 25ss with CD ignition. Maybe the battery is dead in your tiny tach, if it resets the battery is ok? Sometimes you have to press and hold the reset button for 5 to 10 seconds before it will start the reset process.

Is your Tiny tach the old model (1C or 2C) or the new type that allows adjustment for different engine types?

How do you know rpm with the dewald prop if the tach is not working?

What type of device are you using for the engine kick in or out?