Tranny engaging only at higher RPMs is generally a sign of the clutches slipping inside. Caused by cable or lever adjustment that prevents it from engaging properly. Or wear ! Do as Kloudie suggests. Do it in step, i.e, disconnect the cable at the lever on the tranny and have someone activate it from the pedestal while you watch for full movement. Then start the engine and with cable still disconnected, activate the lever manually to see if the tranny engages properly. You need to go out and run long enough to warm the tranny oil up before you do that. When it's cold it almost always works perfectly. When it is hot, the oil gets thinner and the clutch will slip more. If you do not want to go out, secure the boat tightly in its berth and run it in gear at +/- 1200 RPM. It will take longer but it will warm it up enough to test it. Last suggestion, extract all the oil (need to pump it up) and replace with proper oil. The plate on your tranny will tell you what's required. Do not use transmission oil if it calls for straight 30 grade oil. And do not overfill over the mark on the stick. After you've done all that, if it still slips, then you are looking at worn clutches inside the tranny. Which means a rebuild. Keep your fingers crossed. Good luck.
