Re: MPH Gauge doesn't what should i problems should i look into...
Boat speedometers can have their pitot tube connected to a plastic piece that mounts to the transom or through a pipe that runs through the lower leg of the motor. Sounds as though you have the latter and if that is something that you have modified then that's the first place to start looking. If you have an air leak or the pitot tube (the small hole that Gerry refers to) is blocked then that will stop the speedo from working.
Look under your boat's dashboard and find the tube running from the center of the back of the speedometer. The tube probably will be black plastic and thicker than most boat wiring. Trace the speedometer tube to the back of the boat, where it will connect either to a pitot tube under the transom or to the lower unit of the outboard motor. Inspect the length of the tube for holes, cuts, kinks or wire ties that visibly pinch the tube. If the tube is cut or has a hole, replace the tube. If a wire tie is pinching the tube, remove and replace the wire tie, making sure not to pinch the tube.
Disconnect the speedometer tube from the pitot tube or lower unit, either by releasing the quick-connect joint to the motor or by using the screwdriver to unscrew the connector at the pitot tube. Inspect the pitot tube for breaks or holes; if any are present, replace the pitot tube.
If your speedometer tube connects to the lower unit of your outboard motor, open the case of your motor's lower unit and visually inspect the tube for cuts, breaks, holes or kinks. If any are present, replace the speedometer tube.
Tell your assistant to watch the speedometer and let you know if the needle moves, then blow into the tube for 10 seconds. It takes very little breath to make the needle move as high as 5 mph.
If the needle does not move when you blow through the tube, disconnect the tube from the back of the speedometer (it should simply pull off) and use a compressor--at a very low pressure, under 10 psi--to blow any debris out of the tube. When the compressed air blows freely through the tube, reconnect the tube to the back of the speedometer and blow through the tube again while your assistant watches the speedometer. If the speedometer still refuses to move, replace the speedometer unit on the dash. This unit is not repairable by the consumer.
I find that a can of compressed air for cleaning computers works really well for this as you can insert the tube into the speedo pipe and get a good force of air.
If the speedometer tube in your lower unit is damaged, the damage probably resulted from the tube rubbing against the gear case. When you replace the tube, be sure that it is not pushed against the side of the gear case.
