[C320-list] Engine vibration at low RPM. 3GM30

[Brahmany Kite]

The shaft contacting the stern tube is a source of vibration also as there
is not much clearance. Especially if there is misalihnment. On one boat i
found the gland body pushed too far on astern through the hose and bumping
on the inboard end of the stern tube. Yanmar mounts are soft as you know
and small clearances dont help. Under load the engine will twist slightly
sometimes allowing shaft contact as well. The old screwdriver end held to
the ear trick will help find the source. Just a thought.
On 14/05/2015 10:28 PM, "Graeme Clark" <cg at skyflyer.co.uk> wrote:

> Jeff
>
> New mounts fitted as the  rear starboard original had become disbonded due
> to a diesel leak onto the rubber. They were 18 years old original to the
> boat, anyway!  All the correct spec mounts in the correct place!
>
> BUT -  they aren't Yanmar mounts. They are made by a  company called R&D
> as a direct replacement and I chose them because the design protects
> against a  similar problem as they are shielded and the rubber is inserted
> into the mount not bonded to it.
>
> There is a page on the Yanmar site about using non-Yanmar mounts and it
> does mention that “other makes’ can be stiffer which increase vibration at
> lower rpms.
>
> The tacho is spot on, calibration, (I have a laser tachometer that I used
> on the crank pulley to check). So at 1000 rpm its in fine form, at 800 its
> shuddering away!
>
> One thing someone else suggested was to check that with higher amplitude
> vibrations it is possible that some part of the engine actually touches the
> boat; I’ll have to check next time I am on board.
>
> So really my question is whether it is normal for the engine to shake
> around a lot at low revs, rather than for the boat itself to shudder.
>
> The other thing I am unsure of is how the governor works - mine maintains
> a set RPM when the boat is out of gear or if the boat is in gear, but when
> you transfer from neutral to forward gear, obviously the engine comes under
> load, and the rpm drops requiring more throttle.
>
> I don't know if this is expected behaviour or not?
>
> Thanks
>
> Graeme
>
>
>
>
> On 13 May 2015, at 23:24, Graeme Clark <cg at skyflyer.co.uk> wrote:
>
> >> Hi,
> >
> > It depends.  Do you feel boat vibration, or are you just seeing the
> engine
> > "rocking" in its mounts?  If you feel the boat shuddering, then this is
> not
> > normal, and *usually* indicates that the idle is too low.  If the engine
> > "looks" like it's vibrating a lot in its mounts, that's probably ok as
> long
> > as the prop shaft isn't bouncing all over. Some shaft movement is to be
> > expected at idle however.
> >
> > The engine is supposed to oscillate (rotate/rock side to side) in line
> with
> > the prop shaft.  (imagine the prop shaft extending through the engine and
> > out the front and attached to fixed mounts at both ends. The engine will
> > pivot side to side around this shaft, more so when at a low idle, and
> when
> > revved up, it should appear to be much more stationary).  The Yanmar
> rocks
> > quite a bit at idle, but gets more smooth as you increase the speed.
> >
> > So, if the tach reads 1000 and you feel the boat shuddering, the idle is
> too
> > low and your tachometer is probably reading a little high.  Increase the
> > idle by adjusting the governor (on the cable right below the engine
> mounted
> > fuel filter).  If the tach is correct and it only smooths out (stops
> > shuddering) at higher RPM, your engine mounts may be shot.  Another
> thing to
> > notice is if you rapidly back the throttle down to idle position and the
> > engine shudders or nearly stalls then again, your idle setting is too
> low.
> >
> > Cheers!
> > -Jeff Hare
> > #809
>
>