See full version: How to Fix the Speedometer Hole on a Mercury Outboard Motor
Check the tube in the back of the gauge and be sure it is not cut, kinked or tied too tightly for air to move through. If it is cut, you will need a replacement tube. Otherwise, untangle or loosen the wire or kinks and you should be ready to go. more
Follow the tube to the back of the boat. Unscrew it out of the pitat pick-up or lower unit and attach it to an air compressor. Gently blow air through the tube. The needle on your gauge should move. If it doesn't, you will need to buy a new gauge. here
Sailing at the appropriate speed is very important, especially when participating in water sports or when there are swimmers or other boaters sharing the lake. For this reason, keeping your speedometer in good working order is crucial. In most cases, the speedometer works by implementing water and air pressure. As you sail, water is forced into a small hole either in the pitat pick-up or in the lower unit of the motor. The water forces the air in the tube to compress and moves the needle to show your speed. If the hole becomes clogged or damaged you will receive no reading or one that is inaccurate. more
Use the air compressor to dislodge debris from the tube by setting it to blow air with a little more pressure. If you cannot remove it, you can purchase a new tub and simply screw or insert it into your pitat pick up or lower unit.
I know mine is plugged. I bump a log last time out. Dug some wood (rotten) out of it while on the water, but it's still blocked.
If you want to see if the speedometer gauge is actually working, remove it from the dash. Stand in front of mirror and blow with your mouth into the inlet on the back of the speedometer. You should see the needle move a bit. If it does not move, then the speedo may be defective.
Re: repair the speedometer on an outboard
Your outboard has a water pressure speedometer located on the lower unit of your motor. From time to time, this feature will get clogged with debris, weeds and gravel. This problem is easy to fix with one simple tool . a drill bit.
Using the fluted end of the drill bit, slowly slide the bit back and forth to remove any debris.
Find a drill bit that fits appropriately into the hole.
When the pitot tube that provides water pressure information to run your boat's speedometer clogs, it's an annoyance. While the Mercury SmartCraft display is able to extract speed data from a GPS receiver on boats so equipped, if your boat is not equipped with a GPS unit, it will have no source of speed information. You can avoid the fines you might incur from exceeding boat speed limits, where imposed, and keep your speedometer working properly with a compressor and a small piece of wire.
Reconnect the pitot tube before storage.
Run a thin piece of wire through the pitot tube to ensure it's open, and to ensure it drains completely if wet debris was trapped inside.