Half of a Speedometer

[Tregs]

Boat info: 2003 Chaparral 180SS, New 4.3L Mercruiser (2 hrs)

I test drove the boat 2 weeks ago and the speedometer worked fine. I had the dealer I bought it from install a depthfinder in the factory location in the dash. When I took the boat out last weekend, the first time driving it since the depthfinder was installed, the speedo didn't work at all until I got it up to speed. It then began working, but it only registers about half actual speed. I took it out again today....it did the same thing....registering about half the speed.
The dealer will fix it when I take it back in for the 10hr service, but I'd rather fix it now if it's something easy.
Any ideas?

 

[foodfisher]

Re: Half of a Speedometer

Check for debris in the pitot tube entrance,

 

[kmarine]

Re: Half of a Speedometer

I keep a paperclip to poke in the speedo pilot tube. Debris will clog them regularly. You can also remove the tube from the back of the speedometer and blow out the debris. Also see if they kinked the tube when the instaled the depthfinder