I have a 90hp Mercury. It has started to make a real clunky sound when taking it in and out of reverse. Also, when moving at very slow speeds, the motor makes a really deep rumble and seems to vibrate the boat more than before. I just replaced the aluminum prop with a stainless of the same pitch. Could it just need a tune up or could it be the lower unit starting to go out? What are the signs of a troubled lower unit?

first thing i would do would be drain the oil in the foot,see if their is water or metal shavings in the oil,and go from their...

03bluetj

Did you tighten the prop nut too tight? ( not familiar with merc...but on older OMC's you dont wanna do that....it loads the bearings)

Did it do it before you swapped props?

Drain the lower unit oil, like he said. There will be a tiny bit of metal, sure...that's normal.. ( TINY)...but if it's a lot, you have probs.

I don't remember it doing it before I replaced the old prop. I hit a stump at about 5 mph, thats why I had to replace the old prop. It messed it up pretty bad. I Drove the boat for a few miles at low speeds with a bent prop and to get it back to the ramp. At higher speeds the bent prop was causing the moter to vibrate quite a bit. I don't think I hit the stump hard enough to do damage to the lower unit. Could driving the boat with a bent prop for a short amount of time caused damage to the lower unit?

I do have a tendancy to tighten bolts a little too much. I'll back it off a little and see if that takes care of the vibrations. That wouldn't explain the clunkiness I get when putting it in and out of gear though, would it?

could've bent the prop shaft or loosened a gear tooth...take bluetj's advice...don't run it anymore without at least seeing if the gearbox is making metal...gearbox siezes on ya, no telling what it might do to other parts including engine internals.

The drain plug is magnetic & will normal have a little bit of very fine metal on it...when you drain the oil examine it for shavings or splinters - bigger bits - if they're there, tear that puppy down & see what's up in there.