Check for a pitot tube on the leading edge of the outdrive . . . probably clogged.
Best regards, Ted . . . . Cape Cod, MA
Formula 330 Sun Sport, O'Day Mariner Sail #3224, Sunfish Past Boats: Catalina 22 Sail #10531, Formula 242 Sun Sport
Twin Mercruiser 7.4 LX MPI (0F802036, 039), Bravo 3's (0F806198, 199), Mercury 7.5 HP (1969), Johnson 4.5 HP (1980)
As Tpenfield humorously stated...The speedometer pickup for these boats is on the leading edge of the outdrive. (there is no pilot tube sticking out like on other boats) I just had the same issue and had to blast it out with compressed air earlier (the Sacramento river is relentless on debris floating down the river).
Here is a pic of the outdrive edge...you can see the opening:
Here is a pic of the camlock style tubing for the speedometer:
Push the grey button and you can lift the tubing off:
I used some drip line with a fitting on the end to slide into the opening and used the compressed air to blow it out...whatever was in there popped out pretty quickly from the opening on the outdrive. Make sure you do not try to clear it from the outdrive edge up as this would only force the obstruction further up: