Yanmar 2GM Knocking

[WilliamDMoore]

My 1984 Yanmar 2GM has started knocking when it's in forward above 3000 RPM. It doesn't happen in neutral, so I wonder if it's a transmission problem. Any suggestions would be appreciated. It sounds like metal to metal. I hauled it this week. and the prop seems to be o.k.

[JimsCAL]

Sounds like unbalance to me. Were there barnacles or other fouling on the prop when you hauled?

[jimrafford]

Could be the shaft in the stern tube. Check your engine alignment.
JIm

[MarioG]

Dont rule out dirty injectors. Under load it requires more fuel, You get whats called an octain knock if the injectors are dirty and not able to feed the correct amount of fuel.

[WilliamDMoore]

Jimscal - I'm sailing fresh water, and when I hauled, the prop was clean.

Jim Rafford - I checked the engine mounts, and it seems solid. How else would it get out of alignment?

Mario G - I have since looked at other posts, and can't rule out a fuel problem. I did some aggressive sailing this season, and suspect that I churned something up from the fuel tank. I need to find instructions on cleaning/changing injectors etc.

Thanks to all.

[WilliamDMoore]

Oh yes - there was some dark smoke when it was knocking which might also indicate a fuel mixture problem.

[JimsCAL]

The black smoke could indicate overloading. Was the bottom fouled?

What about play in the strut bearing? If worn that could result in knocking.

[Faster]

Originally Posted by WilliamDMoore

Oh yes - there was some dark smoke when it was knocking which might also indicate a fuel mixture problem.

This sounds a lot like a 'bad' injector... try bleeding each injector momentarily when it's knocking... the knock will disappear when you disable the offending one.

What usually happens is the injector fails to atomize the fuel properly and the 'knock' (which sounds alarmingly like metal-on-metal) is caused by trying to compress liquid fuel. If the smoke is more 'brown' than black that's another indicator of this issue.

[WilliamDMoore]

Thanks all

I was talking to another owner with the same engine yesterday, and he suggested an oil additive that has an injector cleaner in it. Does anyone know of one? The boat is hauled, so I won't do anything until next spring. Our season is very short in Canada.

[Faster]

There are various fuel additives that claim injector cleaning properties..