Mercruiser alpha 1 gen 2 Drive -knocking noise while in forward or reverse with muffs

[Bkuz]

Good day all- hope someone can help me- while getting my boat ready for launch - all greased & fluids changed
While Running it at idle with muffs - I noticed a knocking ( metallic) while in forward and reverse, sounds like it's coming from The lower unit- almost like the clutch dogs are not engaging fully, or ratcheting -
Runs fine & smooth at idle- and the noise goes away with a slight increase in rpm- but at low idle it's clear-
I am a well seasoned mechanic- all types of eqmp- heavy eqmpt to cars - boats drives are not my strong point-
If anyone has any helpful information it would be greatly appreciated-
I cked the shift cable adjustments and all seems fine- the noise almost sounds like backlash in prop shaft or the prop hitting something-
Again thank you for any assistance - it's supposed to go in tomorrow am - and maybe with it having resistance on the prop ( once in the water ) noise might stop? Don't know - but don't recall hearing this in past yrs, then again I am at the age of not remembering so well-
Thank you
Bkuz

 

[fabguy1]

Is the drive up, or down when you hear the noise? If its still up its probably the ujoints in a bind.

 

[Bt Doctur]

If you running a SS prop you might head the dogs trying to overrun.nothing abnormal about it . a minor rpm fluctuation and the mass of the prop

 

[Bkuz]

Drive is down - normal operating position- using a 3 blade aluminum prop
It's odd that if I increase idle speed just a little noise stops

 

[Bkuz]

Thank you guys for your replies- is it possible that it's normal ???

 

[UncleWillie]

Thank you guys for your replies- is it possible that it's normal ???

Like Bt Doctur Proposed. ...

If you consider that during each revolution of a 4 cylinder motor; As each cylinder fires, the engine speeds up for part of the revolution and then coasts until the next cylinder fires a half revolution later. The flywheel on the engine is there to dampen out the majority of this variation.
At low idle the clearance in the clutch and backlash in the gearing allows the unloaded prop, acting as an independant flywheel, to get ahead and then fall back in phase with the engine flywheel. The prop speeds up and then slows down also, twice per revolution, and you'll hear two clunks per revolution.
Increasing the Engine speed increases the flywheel effect and the harmonic frequence of the drive train no longer matches the actual speed.
The Vibrations dampen out and become unnoticable.
Once you load the prop by placing it in the water the effect should be completely dampened out.

At least that's the theory!
If it quites out when in the water, we will gladly take the credit. If it still rattles, we'll deny everything. :bolt: