Jeff wrote:
Capt John wrote:


My first question is why are you doing this? I've never heard of
someone doing this before. Their's nothing wrong with checking the
riser for leaks after it's a few years old, and then after that,
but
checking for carbon.... Remember, every time you clean it up and
expose
nice fresh iron it will rust, the iron covered in carbon is
protected.
Are you getting restricted exhaust because of the carbon build up?
And
how do you know this?

The first time it was checked was by a mechanic (at Ashley Marina in
Charleston, SC) who decided it was the likely cause of smoke at high
rpm. Indeed, the mixer was 90% clogged. That was the worst, but
each
time I've done it the engine has run cooler and better. The entire
job is pretty quick, an hour or so for both sides.

I'm not sure there is any iron in the mixing elbow - it seems to be
an
aluminum alloy. Does any know what it its made of?

As for checking for leaks, eventually they will occur, and likely at
an inopportune time. I'd rather do the work a year or two early than

in the middle of a vacation. The last time I had a mixer go it was
the day before a six week vacation. I would have missed it, but a
friend, while doing some other work, happened to tap it wrench and
started handing up large pieces of metal. It surely would have
failed
within a few hours.



It's a good idea to give any engine a good long run from time to
time.
This helps to burn off excessive carbon, this is the preferred
method
of keeping it to a minimum.

I'd guess that about 70% of my run time is at 2600 RPM or higher.
I'll be interested to see how much buildup I've had in the past year,

since I made a point of revving a bit higher last Summer, at about
3000 rpm when I could.

I've never heard of anything like this kind of clogging. I've seen iron
risers start to really rust up as they start to leak. It sounds like
your building up soot, which is mostly carbon, but it's normally caused
by an engine running too rich. In the case of a turbo charged engine
soot is frequently caused by running the engine at the point that the
turbo is not producing enought boost to get the right air/fuel mixture
for proper combustion (too rich). Running the engine at slightly higher
RPM's, so that the turbo is putting out enought boost, usually cures
the problem. A dead give away to this is a dirty transom, the soot is
building up on the transom. You could also have an injector pump
problem, too much fuel. Or the boat could be over propped, does it
reach the rated RPM's at full throttle?