My dash mounted speedo stopped working today.  I'm thinking some mud may have gotten into the speedo tube in the lower unit.  Is it possible to apply air pressure from the speedometer end of the tube to blow out the debris or am I looking at dropping the lower unit?

I don't know about your Merc but on my Yamaha I use a very small drill bit, very small. DO NOT use a drill, only by hand, rotate the drill bit into the hole until debris is removed.

I repeat DO NOT USE A DRILL...............Al

I don't know about your Merc but on my Yamaha I use a very small drill bit, very small. DO NOT use a drill, only by hand, rotate the drill bit into the hole until debris is removed.

I repeat DO NOT USE A DRILL...............Al

thats what ive done before   worked fine      but now i got gps  so i dont care about the speedo        

I haven't had a functioning speedo in years. They're worthless. Terribly inaccurate.

The drill bit worked fine.  My speedo's accuracy wasn't too far off from my GPS.  I like to have a backup just in case.

Thanks for your help.

To answer your question, no, it will not hurt anything if you blow back through the tube.  There is probably a plastic fitting back near the motor connecting the motors tube to the boat's, you can undo it right there.  30 - 40 pounds of pressure won't hurt, that's about what it takes to make the speedo work.  While you have it disconnected, you can slowly apply pressure to the speedo end of the tube and check the speedo if the LU pickup was not plugged.