Boat speedos are inaccurate, and not really worth fooling with. I used to try and keep mine working, but after it got clogged for the 3rd or 4th time, I just left it. No way I'd drop my lower unit to fix this if I were you. I will say that on my old boat the pickup was in the lower unit, and wood got packed into it tight so it was tough to unstop. I used a small drill bit by hand to work the wood out of it.