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Old 09-30-2019, 09:55 PM   #9
Lepke
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City: Between Oregon and Alaska
Vessel Name: Charlie Harper
Vessel Model: Wheeler Shipyard 83'
Join Date: Jun 2016
Posts: 3,023
If the engine is in good condition (good compression) my first guess it's over propped. I don't know Yanmars well, but diesel fuel injection is controlled by a governor usually built into the injector pump. When you advance the throttle you're really changing the speed setting on the governor. It causes more fuel to go to the injectors. When the engine reaches the speed setting the fuel is reduced to maintain the speed setting. If you're getting black smoke, the pump is supplying more fuel to maintain the speed setting than normal. It also means you're asking the engine to supply more hp than it was designed to give. The only other common reason for black smoke is the air fuel ratio is wrong. Too much fuel, not enough air. That can be from some air restriction of intake air - not enough engine room vents, dirty air filter (most marine diesels, especially big ones, don't have an air filter, just a screen to keep things out). It is assumed a marine engine isn't exposed to dirt in the air like land engines. The other cause for air restriction is the turbo turning slow because of a dragging bearing or an exhaust restriction that slows the exhaust leaving, hence not turning the turbo fast enough. The exhaust at high rpm spins the turbo so anything restricting the exhaust slows the turbo. Like a muffler too small, small exhaust hose, partially plugged exhaust elbow, too much water in the wet exhaust.
If it's blue smoke it's oil getting past the rings, bad valve seals, or a leaking turbo oil seal. bad rings show up in a compression test.



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