Yanmar 3gm30f - knocking on start

[carlager]

The Yanmar 3gm30f on our Beneteau Oceanis 36cc makes unpleasant knocks when it is started which make make the shaft tremble and put some pressure on the shaft seal ( a Volvo Black Jack).

The knocking stops when I give it more throttle.

The mechanic says the reason is that the motor needs to be raised by a couple of centimeters

Is this something that ought to be rapaired or should I increase the RPM in idle to avoid the knocking when the motor is started ?

[hobopacket]

Is this something that has developed or from the start when you purchased?

I ask because if it is something that has developed what would cause the motor to lower? Maybe the rubber bushings on the motor mounts?

[carlager]

Originally Posted by hobopacket

Is this something that has developed or from the start when you purchased?

I ask because if it is something that has developed what would cause the motor to lower? Maybe the rubber bushings on the motor mounts?

It has been there for the latest 6 seasons. The mechanic says that Beneteau has used mounts in the front that is too low. The boat was built in 2003

[foreverfreedom]

Originally Posted by carlager

It has been there for the latest 6 seasons. The mechanic says that Beneteau has used mounts in the front that is too low. The boat was built in 2003

I would be tempted to replace the engine mountings - unless someone comes up with a better answer.

[topmast]

Just disconnect the shaft flange, whip out your handy feeler gauge and get grip on the situation. Maybe the rubber mounts need replacing or just adjusting. Either way ,it's something you should know how to do.

[gonesail]

you could also get a second opinion from another mechanic or from a beneteau forum.

[galynd]

I have the same boat and had the same problem. I raised the idle rpm's from 700 to 900. I haven't had the problem since.

[Diesel Bill]

Could it be the idle is to low causing the engine to shake itself to death. Most engines when idling to low will start to shake.

[carlager]

Originally Posted by galynd

I have the same boat and had the same problem. I raised the idle rpm's from 700 to 900. I haven't had the problem since.

Sounds like the right solution. Do you know where there is a guide on how to adjust the idle rpm

[galynd]

Follow your throttle cable where it connects to motor. Screw driver and small wrench, abt 8mm. Pretty simple process.

[captstu]

Jan are engines usually knock when the idles too low.

If the engine knocks at idle when it is not in gear adjusting the engine mounts will have no effect.

Stiffer engine mounts may reduce the knocking someone.

Most likely the item was to raise it a half a turn try Again until 900 rpm or so.

[Scout 30]

If the front of the engine is too low it's out of alignment which is bad for several things but I've never heard of it causing the motor to knock unless maybe you have a bad motor mount. If it's happening at start up you're in neutral so alignment wouldn't come into play but a motor mount might although that wouldn't sound like metal on metal contact. I'd get a stethoscope & try to figure out where the sound is coming from. Might be a bearing.

[roverhi]

My Yanmar knocks if I don't give it enough throttle when starting cold. Give it enough fuel and the knocking doesn't happen and won't occur when throttled back. Yanmar's are cold blooded.

[Chuck Hawley]

My similar engine does a similar thing. I start with very modest throttle, and the sound goes away in a few seconds. I believe some of this is due to a lack of oil in the bearings since you can hear the engine quiet down in about 5-10 seconds. I wouldn't worry about it, and I would advance the throttle to a level which produces 1200 RPM or so upon firing.

[silentwings39]

We have some friends with this motor fitted in an Elan 36. They bought it new in 2002 and noticed the knocking noise getting worse over the first 3 years.
He fitted a flexible coupling because the engine was moving around so much only to find out eventually that the engine mount brackets were loose!

I find it strange that the alignment would be incorrect from new, but friday afternoon boats do exist. Engines of upto 3 cylinders do have a habit of shaking a lot at low revs.

If the engineer has suggested raising the engine by a few centimeters this suggests that it is quite badly out of line, raising engine revs will not cure alignment problems.

As suggested earlier, try separating the flange couplings and see if the gap is equal top, bottom and each side, if out of alignment there will be quite a bit of variation.
If alignment is OK and the noise is just the engine rattling around then raise the idle revs a little to smooth it out, but check the mounts are secure and not delaminated first.