The Yanmar exhaust mixing elbow is a weak spot for the motor.  It clogs and then performance decreases. At one point I couldn’t rev the motor over 1,500 RPM.  I cleaned it by soaking it in Apple cider vinegar which dissolves the carbon deposits but at some point the buildup is just too great.  So I purchased a stainless steel exhaust from a company called HDI Marine.  It’s a direct swap.  Here are pics of the new one as well as the old one.  Once I removed the old exhaust I looked at the water passage and saw that it was nearly 100% blocked!  Easy job…took about an hour taking my time.  While I was down there I adjusted my packing gland which seemed to be dripping quite a bit.  So a bit of maintenance done!

 

Stainless Steel exhaust for the Yanmar 3YM20 engine.
Stainless Steel exhaust for the Yanmar 3YM20 engine.
View inside the stainless exhaust.
Stainless exhaust installed on the Yanmar engine.
Old Yanmar exhaust.
Corrosion and carbon buildup in Yanmar exhaust.
Old exhaust water port was nearly 90% clogged.