Speedometer inop???

I have a 97 Nitro 800 with a mercury 150xr6. My speedometer hasn't been working. In the past there is a pickup hole on my lower unit that has gotten plugged. Anyway I cleaned it and still didn't help. I'm wondering if it's the actual speedometer itself. It's actually stuck at 20 mph. How do I bench test it? Is there a water pressure specification for the speedometer? Thanks for any input!

just my input.

pull the gauge out and throw it away. instal a gauge like an engine water pressure gauge.


but.. to "test" it.. pull the hose off the back of the gauge and see if the needle drops to zero.

Same thing happened with my '92. Each gauge would stop over time and eventually i just replaced them. They were either all stuck or wouldn't be accurate. Solid advice above.