70 hp Merc Vibration

[jd438]

I have '89 Mercury 70 hp motor that I bumped a stump with at 5 mph last fall and bent up my aluminum prop. Put a new prop on it and took it back out. I notice a very slight vibration at low idle when in gear. Go any faster and it goes away. I have really looked at the prop shaft, and I don't think it is bent. What should I do? I don't want to ruin the whole lower unit.

 

[ENSIGN]

Re: 70 hp Merc Vibration

I would suggest having the propshaft checked for trueness using a dial indicator.

 

[Texasmark]

Re: 70 hp Merc Vibration

Bumping a stump at 5 mph may not have damaged anything. If you have no dial indicator, you can rig up something to put on the floor under your prop shaft (prop removed, engine vertical) that can be adjusted to barely touch the shaft....two pieces of 1/4" x 3/4" material comes to mind in hard plastic, steel, alum or the like and placed back at the spline/thread interface (to get out as far as you can to maximize any variance).

Put the nut only on the shaft, shifter in N, and with a wrench slowly rotate the shaft while you lie on your side and watch the clearance between the shaft and gauge. The eyeball can detect .ooo1" degrees of variation in a tight space.

If you pass that, I'd have a prop shop have a look at your prop for symmetry. Oh, did you replace with another alum prop?

But you could be chasing a ghost and the problem could be engine related; just a coincidence that you are having a problem at this time.

Don't rule out a partially sheared woodruff key in the flywheel under the flywheel nut.

Mark