Unusual noise

[North Shore]

I would appreciate some thoughts.....having launched recently my engine is running well, new plugs and no issues in the last four years - hours - approx 370 - Tohatsu 90 3 cylinder two stroke. My problem is when ticking over all is well BUT put into gear and a thump thump thump noise starts, no unusual vibration or 'death rattle' but a steady knocking which increases with engine speed. It still runs as fast as ever and the odd thing is that with the cover off the noise is much reduced. It's almost like the cover somehow emphasises the noise. We've checked the prop isn't loose or moving excessively, there's nothing obviously loose and the engine out of gear is running fine and has a lovely bite as ever I'm off to it with a diagostic mechanics stethescope tomorrow but any clues? I'm thinking gearbox or drive shaft but as engine speed increases the noise is lost in general engine roar etc. There are no metal on metal sounds, I did the impellor last year - totally baffled but also concerned.

[Chris]

What sort of prop does it have, does the prop have an insert (like the quicksilver flo torque...

[North Shore]

I'm not sure....I've not put the prop on.....however the chances are that it was from the local Mercury dealer (it's not stainless, just bog standard). What would this do - are you thinking it's worn or loose?

[Chris]

Sometimes you can get a rattle from the prop that is noticeable at low speed. However if it a pronounced thump that might be something else.

I would be looking at the driveshaft if there ate bushings on the way down the leg perhaps one could be on its way out.

[North Shore]

Many thanks for this, I'll start at the prop and see, interesting to note that my pal tightened it the other day and there was still some movement fore & aft........I'm hoping that with the probe I can be more specific as to where it's coming from but I think you are talking a great deal of sense there.

[jyasaki]

If the prop has enough play to bounce between the nut and the thrust washer, you need a spacer between the nut and the prop. Most likely not the noise problem, though.

Have you tried running the motor on muffs with the prop off and see if the noise continues?

jky

[Pikey Dave]

Originally Posted by North Shore

Many thanks for this, I'll start at the prop and see, interesting to note that my pal tightened it the other day and there was still some movement fore & aft........I'm hoping that with the probe I can be more specific as to where it's coming from but I think you are talking a great deal of sense there.

The fore/aft movement is probably the end float on the shaft, the prop shouldn't move on the shaft, but the shaft will have a few thou movement back & forth.

[Hightower]

Have you tried rotating the prop by hand to see if you can feel/re-create any potential problems? I've had a prop where it rubbed on the gear casing, but it wasn't a thud/thump noise. It's sounds like an expensive noise to me, hope I'm wrong.

You could also try it with the engine running, in gear and prop removed. At least it removes one potential problem component.

[North Shore]

Two issues......firstly the thrust bearing was knackered - see pictures, but unfortunately this did not get rid of the noise so "expensive" is likely correct. The old bearing was oval and thought is that this has caused excessive wear in a gearbox bearing. I have one other question....would a 2002 - 2010 gearbox fit a motor which is likely to be 2000 ish?

[North Shore]

To finish off the thread a replacement lower unit was found, installed and all is well! (except for an over-enthusiastic low water pressure switch which was playing up before)
The advantage of a simple engine is that it's simple to repair and many thanks to all who posted with valuable advice.

On draining the oil out of the old gearbox some very, very shiny pieces of shrapnel appeared and it's grumbling on turning slowly by hand......an interesting winter afternoon project methinks