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

[Elizabeth Schwartz]

Ahoy.....use the screwdriver as a stethescope.....a long one in this 
app....it will magnify any noise when held up to machinery......the but 
end to your ear....Joe...#245 


On Fri, May 15, 2015 at 08:36 AM, Marek Fluder wrote:

> 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 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"  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  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
>>>
>>>
>>
>