Beating Noise while Underway

murdmerc

Cadet
Joined
Feb 12, 2012
Messages
29
I’ve got a 1990 40hp 4cyl 2-Stroke

the boat was last in the water 3 years ago and due to circumstances I’ve not had time for boating until this year so have dusted it off to get going for this season.

the last season I had the boat in the water it started making a deep bassy beating noise while underway. It was a persistent with the rpm and seemed to be in relation to how much load it was under. Wouldn’t do it while reviving in neutral though. This really concerned me and probably part of the reason I’ve not used the boat for so long.

My my initial thoughts were that he crank shaft bearings or something devastating like that was the issue but having read a few things I’ve absorbed the following:

when outboards produce a noise like this when at idle or revved in neutral it is most likely a power head issue crankshaft it similar.

But when symptoms only manifest under load - this then points towards the lower unit. Prop, broken gears etc. Prop is solid and I dings or dents. I’m going to change the LU oil at the weekend so will be looking for large metal pieces on the magnet of course.

after reading a few things I’ve come up with another possible cause. The last season I was using the boat my shift cable snapped - due to age and it was where it was curved at the transom area. I replaced it and I’m now thinking back, this was around the time this beating sound started? I’m wondering if I never setup the shift cable adjustment correctly and I might not fully be in forward gear.

does this sound at all plausible to anyone. Anyone have any experience with this?

boat not in the water yet as still fine tuning bits and pieces so cannot test the theory yet. Just want to know if it’s worth adjusting this?

thanks in advance
 

Faztbullet

Supreme Mariner
Joined
Mar 2, 2008
Messages
15,589
If it acts/sounds like your hitting objects in water adjustment is out and is jumping in/out of gear.
 

w2much

Lieutenant
Joined
Jun 22, 2005
Messages
1,276
If it acts/sounds like your hitting objects in water adjustment is out and is jumping in/out of gear.

Make certain your adjustment is correct on your cable. If lower unit was removed for water pump or any reason the shift shaft may need to be adjusted. Start with your cable as that is what was changed.
 

murdmerc

Cadet
Joined
Feb 12, 2012
Messages
29
Well on Saturday I adjusted my shift cable so that when in Forward position the shift lever is further into the forward gear as My suspicion is that it wasn’t far enough into forward.

things get worse though!

I went to drain the gear oil and man oh man that oil was foul! Stinky - my wife smelled it from the house.
The plug was full of metal shavings and the oil was the dreaded dark metallic grey colour. I’m guessing my lower unit is shot?



i ran some old junk oil through it, I put some automatic transmission fluid in it then ran for a minute it so and drained. Was still dark grey. I then filled with car engine oil ran for a minute and drained. Nice and yellow now. Finally filled with quicksilver gear oil.

I mean my engine runs but I assume my gears are now in shreds.

I’m going to run it this season but think In the meantime I’m going to look for a lower unit on eBay. By the looks of what’s available it’ll have to come from the U.S as none for sale here.
 

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murdmerc

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Joined
Feb 12, 2012
Messages
29
Seems daft but I hadn’t even considered tackling a repair, the lower unit seems like a Pandora’s box which would be beyond me. Have had the powerhead off in the past and changed gaskets etc. and done all the usual maintenance stuff.

How easy is it to get in and fix this thing. Once the lower units off. What next? 😬
 

murdmerc

Cadet
Joined
Feb 12, 2012
Messages
29
Scotland UK

Not sure I’ve got the nerve to tackle a lower unit repair plus seems to be a lot of special mercury tools required to do so.

I’d expect to find that my clutch and forward gears are ground down and I’m guessing the bearings aren’t going to be in great shape with a mixture of gear oil and metal granules lubricating them.
 

racerone

Supreme Mariner
Joined
Dec 28, 2013
Messages
36,048
Well, some of these are easy to take apart for inspection.-----Time to learn or bring out the money.
 

murdmerc

Cadet
Joined
Feb 12, 2012
Messages
29
Thought I’d update this.
I bought a lower unit from eBay £250 oil was black but normal looking and no metal whatsoever on the magnet.

When I pulled the original lower unit off I discovered that although the throttle was in Neutral position the lower unit was positioned 1 spline out of Neutral position and the prop was ratcheting (forward Gear) I don’t know when this happened - was it put together misaligned when I did the impeller 2 Seasons prior? Or was it when I’d been adjusting the shift cable (with hindsight - poorly!) ? This must be the cause of the beating sound, dog clutch semi engaged and I must have did a pretty good job of grinding it all up in there.

Anyway, I Changed the oil in my newly aquired lower unit, new impeller fitted, made sure I was definitely set to neutral! and swapped it out. At the same time fitted new plugs, fuel filter, and rebuilt the fuel pump.

When I had initially removed the fuel pump I noticed there was fuel coming out if the hole on the back which leads to lower cylinder which drives the diaphragm. There not supposed to be fuel here? Then found the lower plug was black and very crusty with severe pitting in the outer metal face - I can now only assume this cylinder was drawing in additional fuel through its link to the leaky fuel pump? (although thinking about it. Is, the channel from the fuel pump is connected to the cylinder under the piston not above it - I think?) so maybe I’m talking myself out of this one now? Unless anyone can suggest otherwise?

Anyway, launched it again and tested today - engine now running lovely no thumping thudding or dying as I’d also had.

i’m struggling priming the engine lately now though. Takes ages for the bulb to get firm. I’m having to squeeze the bulb and hold it squeezed for a bit then release wait a while and repeat 10-15 times it feels like it’s doing absolutely nothing until finally it eventually stiffens up. This used to be much quicker and this is a new line quicksilver 8ft arrangements and its on the right way. Is this a carb issue maybe. When it does firm up if I look at the carbs they both seem to slightly Seap fuel from the throttle pins on each but I wonder is this fairly normal for a freshly primed carb which may be at a higher than normal internal pressure and remedy itself once running?

wondering if although the fuel line is new the one way valve on the bulb is not quite performing?
 
Last edited:

racerone

Supreme Mariner
Joined
Dec 28, 2013
Messages
36,048
The fuel pump is driven by pressure pulses in the crankcase.-----That is below the piston.----You did install a new pump diaphragm I hope.
 

murdmerc

Cadet
Joined
Feb 12, 2012
Messages
29
Yes I rebuilt the fuel pump with a quicksilver kit. Not keen on the small plastic one way valve discs replacing the older 2 piece rubber and plastic ones though.
 
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