Oceanis 36cc - motor knocks on start

[carlager]

The Yanmar 3gm30f on our BeneteauOceanis36cc makes unpleasant knocks when it is started which make make the shaft tremble and put some pressure on the shaft seal ( a VolvoBlack Jack).

The knocking stops when I give it more throttle.

The mechsnic says the motor needs to be raised a little in the front.

In order to do this the mountings in the front needs to be replaced.

I am uncertain if this is a nice or need to do repair.

The knocking can be avoided by giving throttle after the start or increasing the RPM in idle.

Views and tips on this is highly appreciated

 

[Newport Bob]

Have you checked the cutlas bearing?
My thought is if gone the aft end of shaft will be laying down which makes it look like what your mechanic say from engine end. And this would cause the knocking of shaft at low rpm

 

[KD3PC]

the motor mounts may be shot, and that is what is knocking - rather than the engine internals..

It is important that the mounts all work and are in allignment with the prop shaft.

In this case, you may want a second opinion - tough to see over the internet, if you don't trust your mechanic...

Pics??

 

[Newport Bob]

Look at the pics n this post and you will see what I am talking about
https://forums.sailboatowners.com/index.php?threads/cutless-log-replacement.185798/

 

[Bill Roosa]

Had mine do that ALL the time too. It is a 3 cylinder engine operating on 4 cycle. You have it dynamically balanced (crank pins at 120 degrees) and the power strokes every 240 degrees which results in shaking. There is not much you can do to get rid of it either. I just start and idle at a higher rpm and only slow it down to shift (kinda have to as the shifter and throttle are on one stick).
As for the other posters comments, well if it is in neutral then the trans, and shaft, cuttlas...... are not even turning. If it does not shake when you put it in gear AND you have it off idle then you're fine.

 

[viper]

Bill is correct, all 3 cylinder Yanmars knock at idle....and it goes away once warmed up with some more RPM.....

Regards,

 

[helmfam]

My Yanmar 3JH4E, 3 cylinder on my 2007 B 373 knocks and is a bit noisy and vibrates at first on start up too. As Viper stated above if I increase the idle speed just a little it quiets it, and when it is warmed up it does not knock at idle.

 

[Guan]

Please read all the great advice given in my previous query...Engine doing the Elvis.
My 900 to 1000 revs solves the Rick n roll.

 

[braddexter]

Yep.... agree with above. Mine (5 years ago) always knocked and vibrated, warm or not. I increased the idle speed and it went away.