Fisheye,
It's very common for grass, debris,and all sorts of foreign "stuff" to
clog the passage in the lower unit,and as you have eliminated a problem
in the tubing and speedo itself that is the location to suspect.There
is no check valve; if there was the speedo would stay at the max speed
attained and could not return to zero.
Try using compressed air or some water pressure to dislodge the
clog...But always go from the top down;
going from the bottom up will usually result in making things worse,ie.
forcing the clog in even further. I made a fitting up that screws onto
a garden hose and adapts down to a piece of sppedo tubing. Be careful,
just crack the water on slightly, it only takes a couple of psi to
drive a speedo.... You may be able to help things along with a piece of
flexible wire. If no luck, you can always install a stand-alone pitot
tube on the transom and connect it to your existing speedo and hose...
but that shouldn't be necessary...Good luck...
Regards,
markvictor
(visit my group Bad Boat Biz ...a Google group)