speed o meter problems

I have a 2007 240Ex with Mercurier 350 MPI Bravo III inboard
My speed o meter does not work. Someone told me to take loose behind the speed o meter guage on dash and see if any water came through it and if not it was stopped up. So I did and it had no water.
Is the other end of that tube look just like the one in the dash and going into the front side of the prop area????????? I don't see how it works as needs water to enter to guage it. Should I take that black tube off and blow air compressor through it???? any ideas on how to fix it????

Re: speed o meter problems

OK, if you disconnected the rubber hose from the speedometer and did not get any water from the tube when the boat was being operated in the water.....then the speedometer pickup on the out drive is clogged or the hose may be disconnected at the outdrive.

With the boat on the trailer, trim the outdrive fully up. At the front of the outdrive, near the anode, there should be a rubber tube connected to the lower portion of the outdrive. The rubber hose is often overlooked and left disconnected when the outdrive is dropped off when serviced.

The other possibility is, while operating the boat, debris has hit the leading edge of the outdrive and blocked the speedometer pickup hole. Use a long small drill bit to clean out the debris. ONLY turn the drill bit by hand when cleaning out the debris, otherwise a larger problem can be created if a drill is used.

I would avoid using compressed air as compressed air could blow the hose off the outdrive connection. Most likely it won't blow out the debris packed into the pickup hole either. Air pressure dirrected towards the gauge can damage the gauge.

The theory on the speedometer is water enters the speedometer pickup hole on the outdrive. The pressure of the water pushes on the air in the tube which causes the speedometer to deflect. The more speed (and thus more pressure) the more the speedometer needle is deflected. Thus the question of if you have water dripping at the helm. If the connection at the gauge is disconnected, water will enter the speedometer pickup hole on the outdrive and squirt out of the disconnected tube at the helm as you are out running the boat. If the tube is connected to the gauge at the helm, and you simply disconnect the connection at the gauge (and the boat is NOT running on the water) you will NOT have water come out of the hose when you disconnect it at the gauge. Only air should be in the tube at the gauge.