Tracker 50 HP reverse lockdown?

[Mark8076]

I just purchased a small houseboat with a Mercury Tracker 50 HP outboard. When I put it into reverse the motor does not lock down so if I give it any throttle the motor rides up out of the water. I was told they usually lock down automatically?? Do I have to manually lock it?

[boats.net]

It has a lock that locks down on tilt pin(on transom bracket) when you shift into reverse and neutral. Unlocks when you shift into forward gear.