Yanmar 3GM(D) Shakes and Loud - Transmission?

[geoffr]

The other day after picking up the anchor we noticed that at certain RPMs the engine would shake uncontrollably (shakes the whole boat). The RPM at which the shaking happens changes after we have idled the boat for a while, or been in reverse. In reverse the engine has always shaken, now it is worse.

If we put the engine in neutral and rev up the RPMs the engine is quiet (very in comparisson to in Forward or reverse). Since we have an outdrive it can't be a bent prop shaft and I'm doubting missalignement given it is inconsitent about which RPM it hapens at.

Anybody have any ideas as to what is going on? We suspect it is the transmission, but I'm at a loss to explain it. We have a Yanmar 3GM(D) with a Kanzaki KM3A Transmission.

As a side note: ever since we bought the boat when engaging in forward we hear a good "clunk" from the engine. Is this related ?


thanks,
Geoff.
SV Geru - Prout Snowgoose 37' Catamaran

[never monday]

A KM3 is a standard reverse gear. Are you saying you have a saildrive? or drive leg like a Gemini?
Have you checked the motor mounts? what RPM is this happening at?

[geoffr]

No it isn't a saildrive it's a Sillette sonic Type 1 drive leg like the Gemini.

The motor mounts are tight - one was loose, we tightened it first.

The RPMs we see this at change - we've seen it anywhere from 1500 to 2800 (and above we assume, didn't try).

[never monday]

The KM's when they are slipping usually shudder at low RPM and clear as the RPM's increase. What's the exhaust smoke like?

[geoffr]

The exhaust smoke is white (if any), the only time it is black or sooty looking is at startup and when at very high revs.

[never monday]

what color is it during the shaking incidents?

[geoffr]

I haven't noticed what colour it is when shaking - it's certainly not smoking a lot, but since the exhaust is under the bridge deck it could change slightly and we would not notice without specifically looking.
What are you thinking if it is black and if it's not ?

thanks for the help here..

[never monday]

I'm trying to decide if you might be getting a missfire on a cylinder under load. This could explain a heavy vibration, but it should clear at higher RPM.

[geoffr]

By higher RPM you mean above what ? 2500 ? We have seen this vibration at what I would say low and high (2800).

If a cylinder was miss firing would it be consistently at the same RPM range ? Or could it shift about ?
Are there any tests that I could do to check for a missfire ?
Would lifting the decompression lever on a cylinder cause the same thing (so we could see if the "shake" was the same) ?

A test we ran today while going down the ICW was to slowly move from Neutral to about 2800 RPM, then back down to neutral, bit of reverse, then back again - the spots it vibrated at changed each time. I would guess that our speed through the water for each time was about the same.

[Therapy]

What kind of prop do you have?

Have you looked at it recently?

[geoffr]

The prop is a three bladed fixed, I'm not sure of the size. I looked at it this morning to see if had hit something, I didn't see any damage (bent/missing bits etc).

[never monday]

I wouldn't lift a decomp lever. Take a 17mm wrench and crack open the injector line going in the side of the injector.

[geoffr]

Do you think opening the injector line would be worthwhile.. would it tell me anything ?

If you were on site what steps would you take to figure out if an injector was the problem ?

If it wasn't the problem... what's next ?

Thanks again for the help, we are feeling a little stuck here as we want to leave the US trusting our engine

[Stillraining]

FWIW

I have Zero knowlage of that drive unit and a matter of fact I have never seen one before..but it has similarities to the old OMC string drives..and yes any out drive can get a bent prop shaft..I had one 2 years ago as a matter of fact in our SeaRay after striking a stick that wasent more the 3' long and 3" in diamiter..it just caught it just right...2500.00 bucks worth just right...

[Bradley]

Call Sillette Sonic in England. They are very helpful and knowledgeable. I had a smilar problem, and it was the top nut that was working loose. This is part #76. To test you will need a 15/16" socket and an extender.

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