I have the same problem on my 2006 Nissan 8HP (exact same motor as the Nissan/Tohatsu 9.8HP, not sure if this is the same as your 9.9?)- did some research and later models of these motors came with a spring-loaded tilt bracket instead of the manual one. There was technical bulletin from Tohatsu on the change. Your can replace the original tilt stopper with the newer spring-assisted version, part number 3V1624311. You cannot just add the spring, have to get the whole tilt stopper. I'm planning to change mine this winter.
This discussion provides the details- make sure we are talking about the same motor:
Hi all does anyone know if the tilt bracket on the 9.8 mfsa3 should be spring loaded? Its the mechanism with the side handle to latch the motor right out of the water. It automatically locks into shallow water mode (i can see the spring on the bracket) but the full up position isn't locking...
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