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

[JJ Morrison]

I have no vibrations whatsoever from my Yanmar engine and it typically idles at 800 rpm. Depending on how many hours on the engine you might want to have the injectors serviced. We have a diesel shop here that does them for 45CAD, but you have to deliver them. In any event, as has been stated, an engine mount could certainly be the culprit.
JohnM
1999#574

> From: cg at skyflyer.co.uk
> Date: Thu, 14 May 2015 13:28:40 +0100
> To: c320-list at lists.catalina320.com
> Subject: Re: [C320-list] Engine vibration at low RPM. 3GM30
> 
> 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 
>