Vibration at idle

[prayingmantis]

Hello everyone. I have a 1995 spectrum sport with a '96 mercury marine force 120. For the most part runs good. The issue I have is that when I shift into gear and give it a little throttle it starts to vibrate. Then as I give it more it goes away. What could this be ? Is it something I should 've worried about ? Thanks

 

[Jiggz]

Obviously it has something to do with the LU gear housing. When was the last time you change the gear oil in the LU? The other thing is it could just be a very low idle setting. Make sure when in gear idling in the water, the rpm should be at least 750 rpm. And of course, it could also be the prop having imbalance due to corrosion or chipped.

 

[prayingmantis]

I bought the boat back in November because we wanted to run the lower Niagara River all year. I changed the last week. It came out looking like old motor oil. I changed it because I seen some oil coming out from the bottom bolt behind the prop. When I bought the boat it had a stainless steel prop on it. There is no markings on it for the pitch.

 

[jerryjerry05]

Start the motor on a hose.
Put it in gear and watch the prop spin.
If it's out of balance it should show.

​One day I was changing my oil.
Finished the job and just happened to spin the prop.
​Big wobble
Looked at it and saw the hub had slipped and the aluminum was corroded.
Luckily I had a spare.
I never felt anything different when I used that prop.

​It's a wonder it didn't destroy the bearings as I ran it??
​No idea how long it was on there?
I never hit anything so it ust gave out.

​Look under the nut for ID on the prop?

 

[Jiggz]

Check on the driveshaft and the prop for play by trying to move it forward and back and also sideways. There should be no play at all in either directions.