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

[Marek Fluder]

Brahmany,
Can you elaborate on the "screwdriver trick"?
Sounds like some interesting diagnostic.

Marek
#1028

On Thu, May 14, 2015 at 5:49 PM, Brahmany Kite <brahmanykite at gmail.com>
wrote:

> 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
> >
> >
>