I have a Mercruiser 120 fuel problem. There is a small tube going from the
fuel pump to the throat of the carburetor (like a vacuum tube) and it is
injecting a large volume of fuel into the carburetor throat, bypassing the
float assembly altogether and flooding the engine. When I remove this tube
and attempt to plug it, the engine runs properly but there is so much
pressure on this little tube it blows the plug out and starts pouring raw
fuel into the boat! Automotive engines don't have this tube, and I don't
know what it's purpose is. Can anyone offer any help? Thanks!

On Sun, 10 Jul 2005 06:21:02 GMT, "Kevin Jed"
wrote:

I have a Mercruiser 120 fuel problem. There is a small tube going from the
fuel pump to the throat of the carburetor (like a vacuum tube) and it is
injecting a large volume of fuel into the carburetor throat, bypassing the
float assembly altogether and flooding the engine. When I remove this tube
and attempt to plug it, the engine runs properly but there is so much
pressure on this little tube it blows the plug out and starts pouring raw
fuel into the boat! Automotive engines don't have this tube, and I don't
know what it's purpose is. Can anyone offer any help? Thanks!



I think that older Mercruiser 120s had a low pressure mechanical pump.
Is this true of yours?
If so, a small length of fuel pipe, with a blanking plug at one end
(even a wood screw would serve to test) and secured with pipe clamps
should serve the purpose.

Brian Whatcott Altus OK

Thanks for the feedback. It does have a mechanical fuel pump, though I don't
know if it is high or low pressure, and I'm sure I could plug this line with
a fitting, but I wantr to make sure it is fixed right. I have been told this
line is a vent for in case the diaphragm ruptures. Fuel would vent into the
carbeurator instead of spilling into the bilge. This make perfect sense to
me except I have never seen a fuel pump with a ruptured diaphragm that
supplied fuel. I thought a rupture would prevent any pumping action at all,
but it has been suggested to me that they can continue to pump fuel through
the "vent" which is exactly what this one seems to be doing. In any case,
I'm going to try replacing the pump and see what happens. Thanks again for
taking the time to respond to my problem.

"Brian Whatcott" wrote in message
...
On Sun, 10 Jul 2005 06:21:02 GMT, "Kevin Jed"
wrote:

I have a Mercruiser 120 fuel problem. There is a small tube going from the
fuel pump to the throat of the carburetor (like a vacuum tube) and it is
injecting a large volume of fuel into the carburetor throat, bypassing the
float assembly altogether and flooding the engine. When I remove this tube
and attempt to plug it, the engine runs properly but there is so much
pressure on this little tube it blows the plug out and starts pouring raw
fuel into the boat! Automotive engines don't have this tube, and I don't
know what it's purpose is. Can anyone offer any help? Thanks!



I think that older Mercruiser 120s had a low pressure mechanical pump.
Is this true of yours?
If so, a small length of fuel pipe, with a blanking plug at one end
(even a wood screw would serve to test) and secured with pipe clamps
should serve the purpose.

Brian Whatcott Altus OK

Kevin Jed wrote:

I have a Mercruiser 120 fuel problem. There is a small tube going from the
fuel pump to the throat of the carburetor (like a vacuum tube) and it is
injecting a large volume of fuel into the carburetor throat, bypassing the
float assembly altogether and flooding the engine. When I remove this tube
and attempt to plug it, the engine runs properly but there is so much
pressure on this little tube it blows the plug out and starts pouring raw
fuel into the boat! Automotive engines don't have this tube, and I don't
know what it's purpose is. Can anyone offer any help? Thanks!

This is a bit out of my area of professional expertise, but I've had
experience (that included a VERY expensive lesson!) with mechanical fuel
pumps on my own Mercruiser and Chrysler engines...I know just enough
about this to be dangerous.

Inboard marine engines are designed to dump fuel into the oil crankcase
if the diaphragm fails...that's a safety measure, to keep gas--and
especially gas fumes--out of the bilge. That line--which is called a
"sight tube" and SHOULD be clear hose (although it's entirely possible
that someone who knew even less than you do replaced it with black hose)
that lets you see whether anything is going through it. If it's
squirting gas, I'd bet real money that the diaphragm HAS failed. The
lifespan of most diaphragms is only about 5-7 years...if the boat has
sat unused for any length of time, it can be even shorter, 'cuz the
diaphragm can dry out and split when the boat is fired up again. So I
THINK you need a new fuel pump. Fortunately, manual fuel pumps are a lot
less expensive than electric fuel pumps.

But, as I said above, I know just about enough about engines to be
dangerous. So before you do anything--especially before you run the boat
again, get the manual for the engine so you'll not only know how to
install a marine fuel pump correctly, but the rest of what you need to
know about the difference between that engine and its automotive
counterpart. And also talk to a certified Mercruiser mechanic...'cuz you
can get all kinds WAG advice here...but HE'LL have the RIGHT advice.

--
Peggie
----------
Peggie Hall
Specializing in marine sanitation since 1987
Author "Get Rid of Boat Odors - A Guide To Marine Sanitation Systems and
Other Sources of Aggravation and Odor"
http://www.seaworthy.com/store/custo...0&cat=6&page=1

The line most likely once was clear, but is now yellow with age, and not
nearly so transparant to see any fluid flowing in it. However, I have
already decided to replace the fuel pump based on advice from two automotive
mechanics and one Yamaha certified marine engine tech as well as the
postings I have recieve on this and other newsgroups I posted this on. I
believe you're probably correct in that the line is most likely supposed to
be transparant for reasons you stated, so I intend to also replace this bit
of tubing as well. I gotta tell you, though, I do appereciate the responses
I have recieved, and I thank you for taking the time to reply to my little
problem. And, I did learn something about mechanical fuel pumps that 20+
years of shade tree mechanics never taught me - that they can still deliver
fuel even with a ruptured diaphragm, a fact confirmed by all three certified
mechanics confirmed. Thank you once again for your reply.

"Peggie Hall" wrote in message
. ..
Kevin Jed wrote:

I have a Mercruiser 120 fuel problem. There is a small tube going from
the
fuel pump to the throat of the carburetor (like a vacuum tube) and it is
injecting a large volume of fuel into the carburetor throat, bypassing
the
float assembly altogether and flooding the engine. When I remove this
tube
and attempt to plug it, the engine runs properly but there is so much
pressure on this little tube it blows the plug out and starts pouring raw
fuel into the boat! Automotive engines don't have this tube, and I don't
know what it's purpose is. Can anyone offer any help? Thanks!

This is a bit out of my area of professional expertise, but I've had
experience (that included a VERY expensive lesson!) with mechanical fuel
pumps on my own Mercruiser and Chrysler engines...I know just enough about
this to be dangerous.

Inboard marine engines are designed to dump fuel into the oil crankcase if
the diaphragm fails...that's a safety measure, to keep gas--and especially
gas fumes--out of the bilge. That line--which is called a "sight tube" and
SHOULD be clear hose (although it's entirely possible that someone who
knew even less than you do replaced it with black hose) that lets you see
whether anything is going through it. If it's squirting gas, I'd bet real
money that the diaphragm HAS failed. The lifespan of most diaphragms is
only about 5-7 years...if the boat has sat unused for any length of time,
it can be even shorter, 'cuz the diaphragm can dry out and split when the
boat is fired up again. So I THINK you need a new fuel pump. Fortunately,
manual fuel pumps are a lot less expensive than electric fuel pumps.

But, as I said above, I know just about enough about engines to be
dangerous. So before you do anything--especially before you run the boat
again, get the manual for the engine so you'll not only know how to
install a marine fuel pump correctly, but the rest of what you need to
know about the difference between that engine and its automotive
counterpart. And also talk to a certified Mercruiser mechanic...'cuz you
can get all kinds WAG advice here...but HE'LL have the RIGHT advice.

--
Peggie
----------
Peggie Hall
Specializing in marine sanitation since 1987
Author "Get Rid of Boat Odors - A Guide To Marine Sanitation Systems and
Other Sources of Aggravation and Odor"
http://www.seaworthy.com/store/custo...0&cat=6&page=1

Kevin Jed wrote:

The line most likely once was clear, but is now yellow with age, and not
nearly so transparant to see any fluid flowing in it.

When the tube is connected as it should be, another more obvious clue
that gas is going into the crankcase is rising oil on the dipstick--a
symptom I ignored because it was the first time I'd run the boat only
days after having the oil changed 'cuz I thought my "shade tree" had
only overfilled one engine a bit... and a bit of ignorance that cost me
$3000 for a rebuilt 454 longblock installed. Two years later, when I
bought 20 year old "project" boat that hadn't even been started in at
least 5 years, I wasn't taking any chances...the first thing I did was
replace BOTH fuel pumps.

I learned something else from that experience: they give owners manuals
for a reason. Had I ever even bothered to thumb through the engine
manual, I'd have known what "gaining oil" meant--or at least known to
smell the oil for gas in it..'cuz that Mercruiser manual warned about it
in no fewer than 4 places. I also partially blame the "shade tree"
mechanic I'd used for routine things like oil changes 'cuz he was half
the price of the yard...had he been paying ANY attention when he changed
the oil, he'd have smelled gas in it. Had I paid the yard to change the
oil and they'd missed it, I'd have had some recourse.

So I learned two very expensive lessons from that experience: 1. yes, we
DO need "no steenkin' manuals"...READ the damn things! 2. Yards aren't
always more expensive.

--
Peggie
----------
Peggie Hall
Specializing in marine sanitation since 1987
Author "Get Rid of Boat Odors - A Guide To Marine Sanitation Systems and
Other Sources of Aggravation and Odor"
http://www.seaworthy.com/store/custo...0&cat=6&page=1
http://shop.sailboatowners.com/detai...=400&group=327

On Mon, 11 Jul 2005 04:00:04 GMT, Peggie Hall
wrote:

Kevin Jed wrote:

I have a Mercruiser 120 fuel problem. There is a small tube going from the
fuel pump to the throat of the carburetor (like a vacuum tube) and it is
injecting a large volume of fuel into the carburetor throat, bypassing the
float assembly altogether and flooding the engine. When I remove this tube
and attempt to plug it, the engine runs properly but there is so much
pressure on this little tube it blows the plug out and starts pouring raw
fuel into the boat! Automotive engines don't have this tube, and I don't
know what it's purpose is. Can anyone offer any help? Thanks!

This is a bit out of my area of professional expertise, but I've had
experience (that included a VERY expensive lesson!) with mechanical fuel
pumps on my own Mercruiser and Chrysler engines...I know just enough
about this to be dangerous.

Inboard marine engines are designed to dump fuel into the oil crankcase
if the diaphragm fails...that's a safety measure, to keep gas--and
especially gas fumes--out of the bilge. That line--which is called a
"sight tube" and SHOULD be clear hose (although it's entirely possible
that someone who knew even less than you do replaced it with black hose)
that lets you see whether anything is going through it. If it's
squirting gas, I'd bet real money that the diaphragm HAS failed. The
lifespan of most diaphragms is only about 5-7 years...if the boat has
sat unused for any length of time, it can be even shorter, 'cuz the
diaphragm can dry out and split when the boat is fired up again. So I
THINK you need a new fuel pump. Fortunately, manual fuel pumps are a lot
less expensive than electric fuel pumps.

But, as I said above, I know just about enough about engines to be
dangerous. So before you do anything--especially before you run the boat
again, get the manual for the engine so you'll not only know how to
install a marine fuel pump correctly, but the rest of what you need to
know about the difference between that engine and its automotive
counterpart. And also talk to a certified Mercruiser mechanic...'cuz you
can get all kinds WAG advice here...but HE'LL have the RIGHT advice.


This post of Peggie's hit the jackpot, for me. It was insightful
(about not letting gasoline dribble) and confirmed for me what the OP
finally concluded_ a neat gas feed into a carburettor is a very very
bad thing!

Choking has a place, but over rich fuel does an engine a power of no
good. I am coming to the conclusion thjat a prime reason that modern
engines with computer fuel control, timing etc. last so much longer
than back in the day, is that the gas/air mix is always optimal to
lean - which works well if pinging is also avoided.

Brian Whatcott Altus OK

if the pump malfunctions it will put the fuel in the engine
and not in the bilge
"Kevin Jed" wrote in message
m...
I have a Mercruiser 120 fuel problem. There is a small tube
going from the
fuel pump to the throat of the carburetor (like a vacuum
tube) and it is
injecting a large volume of fuel into the carburetor throat,
bypassing the
float assembly altogether and flooding the engine. When I
remove this tube
and attempt to plug it, the engine runs properly but there
is so much
pressure on this little tube it blows the plug out and
starts pouring raw
fuel into the boat! Automotive engines don't have this tube,
and I don't
know what it's purpose is. Can anyone offer any help?
Thanks!