Jeff wrote:
Every few hundred hours I've pulled the exhaust elbows from my
2GM20F's to chip the carbon away. The starboard engine in particular

has a tight turn in the exhaust hose, and since it gets used some at
low rpm for charging, it tends to get some buildup.

I've been thinking that its time to replace the elbows this year.
Here's my question: The mixing elbow, which is made of some light
alloy and thus is wearing down, is screwed onto to the exhaust elbow
(cast iron?) with a connecting nipple. Since the iron parts seem to
be holding up well, can I simple replace mixing elbows, or should I
do
the whole system. This would be an easy decision if I though I could

get them apart without destroying them, but I've assumed this would
involve a large bench vise, oversized pipe wrench, and a breaker bar!

Also, the cost is not trivial - the mixing elbows are about $100,
but the exhaust elbows plus the nipples are another $125. (All times

2, of course!)

Plus a follow up question - does anyone know a good source for parts
like this? I tend to use Marine Diesel Direct (Torreson) and I've
also seen OK prices at shipstore.com, but does anyone know of low
cost
parts supplier, perhaps in the Boston area? The major distributors,
like Mastry tend to quote list price.

Here's two pics I took a year ago - obviously it hasn't gotten
better!
http://www.sv-loki.com/img_0793.jpg
http://www.sv-loki.com/img_0794.jpg

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?

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.

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.

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?