Tohatsu outboard whine ??

[Nineteen]

Hi
We have 2 9.8 short shaft 2 stroke outboards for our RIB. 1 does it and one doesn't and I have not been able to pin it down. I would describe it more as a howl but it has the same characteristics as you describe. I have checked gearing and even played with prop pitch. A change in load or pitch just seems to move the range without changing the amplitude of the noise.
I was starting to think that it is intake resonance, perhaps even the reed valves. Now that you have a 4 stroke doing it as well has me scratching my head again.
Both my 9.8s are a similar build vintage but had different lives before we got them. The more used one is making the noise. It really seems to be coming from the head and not the leg so I think gearing is out in our case. They both have good compression and are smooth to pull through. I have just got a stethoscope for another project so I will re purpose it next time I take it out.
It only happens under load so I cannot duplicate it at home.
Cheers

[Jef & Marin, Netherlands]

We had a Tohatsu 8hp four stroke from 2012 that had a whine from new. It occurs at RPM a bit over tickover, under light loads; for instance when reducing speed.
I complained under warranty, and was told it is intake induction noise. I was given a propeller with a bigger pitch. This reduced it.
Since 2014 we have a 9.8 four stroke which is basically the same design. It has exactly the same problem: under light loads when reducing speed.
I stopped worrying about it.
We now use the engine since more than 9 years and have had the following issues besides the whine:
- Spark plugs fail suddenly. Good to take a spare plug and tool for longer distances

- The prop shaft seal started to leak, replaced

- The engine suspension rubbers failed, replaced
- Some corrosion issues; not too bad since when I bought it I took almost all bolts out and re-installed them with grease; and used Waxoyl especially under the engine cover.

[krissteyn]

Jeff & Marin ... Thanks for reply...
Prop with a bigger pitch almost makes sense - but yesterday I had an epiphany...
My original prop busted , and I had it pinned - the job wasn't neat and left butts on the spline section , which I smoothed out as best as possible...
When I put it back on , I had to push it really hard with a rubber mallet.
Now it sits very securely on the spline.
And , whine has gone 🤩
So , it is possible that a loose fitting prop is sending some kind of vibration up and causing the noise...
It's only day 2 , so I'm not firmly convinced yet...
I'll try it on the 9.8 with some shimstock I have...

[Jef & Marin, Netherlands]

Hi Kris, I will keep this possibility in mind. It however happens both with original and replacement propellers.
Cheers

[wsmurdoch]

Could the noise be coming from the propeller rather than the lower end?
https://www.hydrocompinc.com/wp-cont...Propellers.pdf

[krissteyn]

Originally Posted by Jef & Marin, Netherlands

Hi Kris, I will keep this possibility in mind. It however happens both with original and replacement propellers.
Cheers

If the tolerances are similar , then the prop will still have some play...
In my case the burrs from the machining were enuff to reduce any play to zero...
As I said , I'm not convinced yet, but it's a cheap fix if that's the problem 👍