The engine will run fine with the mixing elbow plugged to a point. The mixing elbow problems on the Yanmar's are usually with the
water injection port not the actual exhaust
passage clogging up. Supposedly you can't clean the mixing port out on the elbow. At least that's what's been the
advice that I've heard. The elbow runs less than $200. Replacing it is relatively straight forward if you can get it off. I had to call in a mechanic to remove the
3GM elbow on mine because of the excessive force required to unscrew it. Took a humongous vice, large pipe wrench with cheater bar and liberal use of the 'blue wrench', (acetylene torch) to get the threads to break loose. It was a PO install so don't know whether they used pipe dope or Never Seize on the threads but the the combination of exhaust and
salt water had corroded it in place. You'll need a new
gasket if you take the exhaust pipe off so order or buy one before disassembling.
Apparently the mixing elbows clog up quickly on the Yanmars if they are run at idle for regularly or for long periods of time. The elbow on the
water injection on my
2GM, looks the same as the one on my 3GM, plugged up with a little over 200 hours on the engine. Owners were elderly and didn't use the
boat much and then had title issues when they died. Sat for several years with the engine being run at the
dock occasionally to keep it from seizing.
The air cleaner filter in Yanmar's is foam and it breaks down over time. GJordon's
advice to check the air filter would be a good idea.
Having said the above, I'd look at taking the boat out and running the engine at cruising
rpm for at least an hour and see if the exhaust smoke still is happening. Might be just caused by lack of use and a good run will clear out what is causing the too rich fuel mixture. A plugged mixing elbow could cause a problem but a too rich mixture is usually on the intake side of the equation. Assume you have the right prop on the boat as too steep a
pitch or diameter will cause the engine to smoke at higher rpm's and not allow the engine to get to maximum
rpm.