Mercury 100hp 2 stroke - off throttle rattle from midsection

[Hydrasports1750CC]

Long story short, ate up up a piston last summer due to poor carb maintenance.
Stripped the motor to the block and installed a new piston, engine runs great.
Roughly 20 hours on it + or -.

Now the motor has a distinct metallic rattle coming from the midsection / gearcase which occurs when the motor is off-load but at speed. This rattle occurs when slowing down gradually from planing speed and is intermittent. It does not occur when the engine is under load.

This noise is not emanating from the powerhead as i cannot replicate it in any way without the boat being under load.

All bolts / fasteners on the motor have been checked, tightened and rechecked and I am running an aluminum solas prop with the hub locked down as tight as it will go.

Any advice is appreciated, I am nearly to the point of vibration dampening the drive-shaft with a pneumatic rubber hose but it just doesn't seem necessary.

 

[Chris1956]

Re: Mercury 100hp 2 stroke - off throttle rattle from midsection

If you are decellarating quickly, the prop could be spinning faster than the forward gear. When this happens the prop will rachet. Test it out. With motor off, put it in fwd. Now turn prop CW and listen for rachet. Is that it?

 

[Hydrasports1750CC]

Re: Mercury 100hp 2 stroke - off throttle rattle from midsection

Tried what you suggested but it's not the same sound.

The rattling noise occurs whenever the load on the motor is reduced. Any reduction in throttle or other change in loading of the engine / mounting bracket causes the noise. Rapid deceleration seems to make it more pronounced.

The noise sounds like it's a rotating piece that's loose but is not spinning at the same speed as the engine.

Understandably these are vague descriptions but I'd prefer not to tear the whole motor apart looking for a rattling noise.

 

[Chris1956]

Re: Mercury 100hp 2 stroke - off throttle rattle from midsection

Piston slap? If you gun the motor for an instant in neutral and then reduce throttle, does that make is sound off? If so, that maybe does norrow it to the powerhead.

 

[Hydrasports1750CC]

Re: Mercury 100hp 2 stroke - off throttle rattle from midsection

The noise isn't heavy enough to be a piston going bad, I got to hear that sound first hand before i had to tear the engine apart over the winter. I took the boat out today and spent quite a bit of time listening to the noise, it's certainly coming from below the powerhead.

Would there be some way for the lower unit clutch or reverse gear to rattle in such a way that it creates a metallic clanking noise?

The strange thing about it is that the noise is on a different frequency than the normal engine noise. This leads me to believe it's something either in the top of the foot or in the mid section that's loose.

 

[Gomer50]

Re: Mercury 100hp 2 stroke - off throttle rattle from midsection

Could be the driveshaft bearing under the waterpump??? Just a guess..