Black smoke under power
- Thread starter mellis18
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how does it run when not under load, but at a high rpm?.... black smoke with a vibration could indicate a bad injector or slight loss of compression in one cylinder.... its getting too much fuel to burn properly....
with the engine running, loosen the nut holding the injector lines on the nozzles. one of them will make a big difference in the running of the engine when you let the fuel pressure leak off, and the other one will make very little difference in how it runs... that will be the bad one.
pull the nozzle and check to see if the tip is still there. sometimes the tip will break off and let the fuel slobber in rather than squirt in. this will cause black smoke and a viberation...
with the engine running, loosen the nut holding the injector lines on the nozzles. one of them will make a big difference in the running of the engine when you let the fuel pressure leak off, and the other one will make very little difference in how it runs... that will be the bad one.
pull the nozzle and check to see if the tip is still there. sometimes the tip will break off and let the fuel slobber in rather than squirt in. this will cause black smoke and a viberation...
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Black smoke is usually caused by engine overloading. Like the other said prop fouling is very often the cause. You could also try putting the engine in neutral and hand spinning the shaft to see if it feels like it is turning easily. The vibration is another clues that your prop is fouled. Good luck
Capt. Wayne Canning, AMS
www.projectboatzen.com
Capt. Wayne Canning, AMS
www.projectboatzen.com
Re: Black smoke
went through all of this last year on my 3 cylinder , checked prop cleaned bottom changed fuel and in the end found that engine was overloaded at high rpm when under load , backed off RPM a little and smoke disappeared....small motor big boat
went through all of this last year on my 3 cylinder , checked prop cleaned bottom changed fuel and in the end found that engine was overloaded at high rpm when under load , backed off RPM a little and smoke disappeared....small motor big boat
If your mixing elbow is foul (usually the inverted U-type), you'll get black smoke like what you describe. Under no load, engine will run fine usually. However, if the foul in mixing elbow is very sever, then it'll result in vibration as well. Also check to see if engine compart has some black soot. If yes, then quite certain it is due to back pressure from mixing elbow fouling forcing the soot to back up to air intake silencer/filter to compartment.
Foul prop can also cause black smoke due to overloading of the engine.
Foul prop can also cause black smoke due to overloading of the engine.
Cheap and easy to check the prop/shaft.
The mixing elbow is a little more of an ordeal to check and replace. Shop around for the new elbow. It is a job to get the exhaust manifold seperated from the elbow. The coupling that joins them has right hand and left hand threads. The problem is usually the rust & corrosion. A torch and some very large pipe wrenches with breaker bars will help.
The mixing elbow is a little more of an ordeal to check and replace. Shop around for the new elbow. It is a job to get the exhaust manifold seperated from the elbow. The coupling that joins them has right hand and left hand threads. The problem is usually the rust & corrosion. A torch and some very large pipe wrenches with breaker bars will help.
I am marching in line here; If not the prop, then probably the elbow.. When the elbow plugs, the high back pressure keeps it from being able to suck in enough fresh air to burn all the fuel.. so ya get black smoke.. but do make sure the prop is clear before digging into the elbow..
Good tip and points by Centerline and Ken13559
So maybe before one takes on the chore of an elbow re-do, what about changing a potentially carbed up air/silencer filter and see if more air getting thru might solve matters or further confirm the status quo and the likelihood of the carbed up elbow.
So maybe before one takes on the chore of an elbow re-do, what about changing a potentially carbed up air/silencer filter and see if more air getting thru might solve matters or further confirm the status quo and the likelihood of the carbed up elbow.
You most likely have too much prop. It doesnt matter the size of the engine, if it cant reach full rpm its going to work harder than its designed and smoke.
Just an FYI for those not familiar. The governor cuts back fuel as rpm nears peak speed. The engine only needs a fraction of the fuel in neutral, that it needs under full load.
Choking the exhaust, as when the elbow becomes clogged, has the same effect as restricting the intake and starving the engine for air. The governor is still injecting the same amount of fuel, but without adequate air (oxygen) it runs rich and smokes/makes soot.
Fouling the prop places greater load on the engine. If the engine is propped correctly, any additional load should pull the RPM down, and you would note the loss at the tach. As speed drops due to excess load, you would begin to see smoke/soot. Its not smoking because its starved for air necessarily, though it has the same effect because as the motor turns slower, its pumping a lower volume of air, but the pump is still injecting a full load of fuel.
Any time the engine works harder than it was designed for, or runs at a poor mixture, it will run different and feel different. All engines vibrate, and a twin 4 stroke inline is a shaker simply by design. And change will be felt. A fouled prop can do a lot of weird stuff, and all the above apply.
Choking the exhaust, as when the elbow becomes clogged, has the same effect as restricting the intake and starving the engine for air. The governor is still injecting the same amount of fuel, but without adequate air (oxygen) it runs rich and smokes/makes soot.
Fouling the prop places greater load on the engine. If the engine is propped correctly, any additional load should pull the RPM down, and you would note the loss at the tach. As speed drops due to excess load, you would begin to see smoke/soot. Its not smoking because its starved for air necessarily, though it has the same effect because as the motor turns slower, its pumping a lower volume of air, but the pump is still injecting a full load of fuel.
Any time the engine works harder than it was designed for, or runs at a poor mixture, it will run different and feel different. All engines vibrate, and a twin 4 stroke inline is a shaker simply by design. And change will be felt. A fouled prop can do a lot of weird stuff, and all the above apply.
We had blocked exhaust elbow from too much "soft' motor use - the engine needs a run at high revs every so often. the effect sounds like what you are describing and as it gets more blocked power is eventually lost.
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