Thump when idleing down

the last time I took my boat out, i noticed a "thump" when idling down. it's a mercury 9.8 1984 model and I haven't been able to determine what it may be. i am sure it is somehting bad, but how bad? it doesnt do it all the time, but it did it 2 times when idling down very low.

thanks for the help in advance!!!

Joe

Check your prop nut, make sure it is torqued down tight. If the prop has play take it off and check your hub inside the prop, you may have torn or broken something loose in there.

Is it doing it when you come off a high RPM to idle or if you do it slow? Are you sure it is coming from the motor, I had a buddy that had a sporatic bumping and finally realized it was is anchor hanging over the front edge bumping every so often, he chased that sound for a while. #-o

Bufford

I guess it happens from idle to low idle after shifting it into neutral. It doesn't seem to do it in gear.

You may simply be hearing the gears engaging and disengaging when you shift.