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!