Power back rattle

[udy6636]

Hello all.

My 93 120hp Johnson started to make some loud rattling noises when i would power back,and over temp horn sounded and then shut down.. so on further inspection one cylinder boar and piston were very badly scored, thinking this was piston slap noise causing the rattle the power head was boared, new Johnson piston and rings installed and oil injection system removed. now on 25:1 for 20 hrs.
On breaking in by the book, hour 2 running at 4000rpm when i throttled back in comes the rattle.. bugga me.. It wont do it at 3500 rpm or less when i throttle back , only at 4000 rpm and i guess higher..havent got to wot stage of run in yet!
As for the first hour of run in all was good and was running very smooth, runs smooth at all rpms so far.
A mate tells me this could be what he calls "prop rattle".with deceleration the foward movement in water drives the prop unloading the drive.

Any help would be great.

 

[kenmyfam]

Re: Power back rattle

How deep into a rebuild did you go ??? Just a thought but it sounds like it could be crankshaft bearings. This is just an opinion though, please wait for other responses from the more experienced out here.

 

[Texasmark]

Re: Power back rattle

Yesterday I described this in detail on the Merc or Outboard forum. A guy with a 4 stroke was hearing the same kind of noises. Since I have been living with it since Dec '05 I have done my homework.

Not currently familiar with OMC gearboxes as I was talking about a Merc, but since the theory applies to the physics of the process, OMC probably is built the same way for the same reason. Merc calls it "Clutch Dog Rattle" and it's normal, just seldom heard. It happens every time you dump the throttle faster than the prop can follow it.

Course I'm assuming that your prop nut is tight and gearbox is in proper working order.

On my Merc, a lower pitched, lightweight prop, reduces it to inaudible levels.

Mark