Black smoke is often caused by two things: First, too much fuel getting into the engine cylinders... second, running under too high a load—often caused by a fouled prop.
Since you've replaced the injectors and such, I am guessing the issue is a load-related one, not a fuel related one. Have you checked to see that the prop shaft is clear of any line wraps and that the prop itself is clean?
Also, you said you were using a Max prop, which is a self-feathering prop. Yes, self-feathering props can change pitch—that's kind of the definition of a self-feathering prop... and if they're not properly maintained, can cause this kind of problem by not working properly.
If something is wrapped around the prop shaft or prop or jammed in the max-prop's feathering mechanism, it doesn't necessarily have to cause vibration of any sort. Diving on it or hauling the boat and doing a visual inspection is the only way to tell.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.
—Cpt. Mal Reynolds, Serenity (edited)
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