Hi Peter, You might want to remove your mixing for a look / see. This clogging-up / carbonizing of the mixing elbow thing is for real. Here's a picture of mine before replacement compared to the brand new unit a couple weekends ago : https://www.flickr.com/gp/133565480@N04/230jwA
As you can see both the exhaust and water outlet side were more than 60% plugged. In my case, debris from a burned-out impeller clogged the water jacket innards completely. There was no way to get any water through from a garden hose and a small sliver could get through installed on the engine. Also, 3-4 tubes were plugged-up in my heat exchanger (I have a 3GM30F) which required running a wood dowel through. In my case small rubber fragments clogged the heat exchanger, no amount of flushing with whatever chemical would have cleaned it. I was told not to worry too much about my elbow from other lake guys.. "More of a salt water thing" they said. Not true. The elbow should be de-installed / inspected / cleaned every 250 hours according to Yanmar. Running the engine at 80% load helps but it will carbonize over time no matter what. As for judging the flow by just looking at / hearing the exhaust I concur. It's almost impossible to get an accurate picture as the reduction is very gradual and your perception adjusts accordingly. Over time, a small fraction of the full flow will likely still appears "Normal". This is especially true for boats with the reverse transom stern, as the exhaust is submerged while motoring and you can't see it. Best of luck -Francois Rivard 1990 34+ "Take Five" Lake Lanier, GA
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