Took my 1995 20' Crownline with Mercruiser 5.7 Thunderbolt Ignition to
Lake Tahoe over the weekend where the air is very thin and had a little
trouble. Could only start the engine by advancing the throttle all the
way. The engine started everytime but stalled most of the time when set
to idle. The engine runs fine at low altitudes. Any suggestions would
be appreciated. Bob J.

what's the elevation of Tahoe? May require carb adjustments.


wrote in message
ps.com...
Took my 1995 20' Crownline with Mercruiser 5.7 Thunderbolt Ignition to
Lake Tahoe over the weekend where the air is very thin and had a little
trouble. Could only start the engine by advancing the throttle all the
way. The engine started everytime but stalled most of the time when set
to idle. The engine runs fine at low altitudes. Any suggestions would
be appreciated. Bob J.

wrote in message
ps.com...
Took my 1995 20' Crownline with Mercruiser 5.7 Thunderbolt Ignition to
Lake Tahoe over the weekend where the air is very thin and had a little
trouble. Could only start the engine by advancing the throttle all the
way. The engine started everytime but stalled most of the time when set
to idle. The engine runs fine at low altitudes. Any suggestions would
be appreciated. Bob J.


It's a common problem with big changes in altitude. There is an idle stop
adjustment screw on the throttle linkage right at the carb. You'll need to
turn that screw in a bit to raise the idle stop of the carb. This is
essentially the same thing as keeping a tad bit of throttle applied at idle.
This will at least allow the engine to idle at high elevations. But you
will still experience a significant drop in power, which will likely need to
be compensated for by a drop in pitch of your propeller. Also, if you
intend to run at that altitude for an extended period of time, the carb
should be re-jetted for that altitude.

RG wrote:
wrote in message
ps.com...
Took my 1995 20' Crownline with Mercruiser 5.7 Thunderbolt Ignition to
Lake Tahoe over the weekend where the air is very thin and had a little
trouble. Could only start the engine by advancing the throttle all the
way. The engine started everytime but stalled most of the time when set
to idle. The engine runs fine at low altitudes. Any suggestions would
be appreciated. Bob J.


It's a common problem with big changes in altitude. There is an idle stop
adjustment screw on the throttle linkage right at the carb. You'll need to
turn that screw in a bit to raise the idle stop of the carb. This is
essentially the same thing as keeping a tad bit of throttle applied at idle.
This will at least allow the engine to idle at high elevations. But you
will still experience a significant drop in power, which will likely need to
be compensated for by a drop in pitch of your propeller. Also, if you
intend to run at that altitude for an extended period of time, the carb
should be re-jetted for that altitude.

The engine has fuel injection. I am interested in why it was necessary
to apply full throttle to start it. I understand that fuel injection
should adjust itself? There was some power loss, but not too much.

RG wrote:
wrote in message
ps.com...
Took my 1995 20' Crownline with Mercruiser 5.7 Thunderbolt Ignition to
Lake Tahoe over the weekend where the air is very thin and had a little
trouble. Could only start the engine by advancing the throttle all the
way. The engine started everytime but stalled most of the time when set
to idle. The engine runs fine at low altitudes. Any suggestions would
be appreciated. Bob J.


It's a common problem with big changes in altitude. There is an idle stop
adjustment screw on the throttle linkage right at the carb. You'll need to
turn that screw in a bit to raise the idle stop of the carb. This is
essentially the same thing as keeping a tad bit of throttle applied at idle.
This will at least allow the engine to idle at high elevations. But you
will still experience a significant drop in power, which will likely need to
be compensated for by a drop in pitch of your propeller. Also, if you
intend to run at that altitude for an extended period of time, the carb
should be re-jetted for that altitude.

The engine has fuel injection. I am interested in why it was necessary
to apply full throttle to start it. I understand that fuel injection
should adjust itself? There was some power loss, but not too much.

Tahoe is aprox 6200ft.

--Mike

"Lost In Space/Woodchuck" wrote in message
...
what's the elevation of Tahoe? May require carb adjustments.


wrote in message
ps.com...
Took my 1995 20' Crownline with Mercruiser 5.7 Thunderbolt Ignition to
Lake Tahoe over the weekend where the air is very thin and had a little
trouble. Could only start the engine by advancing the throttle all the
way. The engine started everytime but stalled most of the time when set
to idle. The engine runs fine at low altitudes. Any suggestions would
be appreciated. Bob J.

The engine has fuel injection. I am interested in why it was necessary
to apply full throttle to start it. I understand that fuel injection
should adjust itself? There was some power loss, but not too much.

Fuel injection does not "adjust itself." You need to give it more throttle
because there is much less oxygen by volume at ~6200ft. Opening the throttle
not only adds more fuel (via the injectors), but more air...then it starts.
The adjustment needs to be made because at idle, the mixture is too rich
(too much fuel, not enough O2), and the motor stalls (or won't start). The
power loss is due to an overly rich mixture. Airplanes have a mixture
control that is adjusted as they ascend or descend for exactly the same
reason.

--Mike

wrote in message
oups.com...

RG wrote:
wrote in message
ps.com...
Took my 1995 20' Crownline with Mercruiser 5.7 Thunderbolt Ignition to
Lake Tahoe over the weekend where the air is very thin and had a little
trouble. Could only start the engine by advancing the throttle all the
way. The engine started everytime but stalled most of the time when set
to idle. The engine runs fine at low altitudes. Any suggestions would
be appreciated. Bob J.


It's a common problem with big changes in altitude. There is an idle
stop
adjustment screw on the throttle linkage right at the carb. You'll need
to
turn that screw in a bit to raise the idle stop of the carb. This is
essentially the same thing as keeping a tad bit of throttle applied at
idle.
This will at least allow the engine to idle at high elevations. But you
will still experience a significant drop in power, which will likely need
to
be compensated for by a drop in pitch of your propeller. Also, if you
intend to run at that altitude for an extended period of time, the carb
should be re-jetted for that altitude.

The engine has fuel injection. I am interested in why it was necessary
to apply full throttle to start it. I understand that fuel injection
should adjust itself? There was some power loss, but not too much.

Fuel injection does not "adjust itself." You need to give it more throttle
because there is much less oxygen by volume at ~6200ft. Opening the
throttle not only adds more fuel (via the injectors), but more air...then
it starts. The adjustment needs to be made because at idle, the mixture is
too rich (too much fuel, not enough O2), and the motor stalls (or won't
start). The power loss is due to an overly rich mixture. Airplanes have a
mixture control that is adjusted as they ascend or descend for exactly the
same reason.

--Mike

A modern FI system does "adjust itself" to provide altitude compensation by
use of the MAP sensor input to the computer (MEFI module). Lower barometric
pressure at high altitudes will cause a drop in manifold pressure, which
will be detected by the MAP sensor and cause the computer to adjust the
injector pulse width. Some new FI systems add a fuel pressure sensor to
provide more accurate altitude compensation. Mercury now uses their own
proprietary FI system, but in 1995 it would be a GM system.

Snowmobilers riding in the mountains have had the altitude compensation
problem for years, until FI came along and solved the problem.

John,

You're absolutely right. I didn't think that a '95 had the ability to
compensate. Thinking about it, '95 aint that old. It's still manual in
airplanes (piston)...I wonder why?
Thanks for the info.

--Mike

"John Wentworth" wrote in message
. ..

Fuel injection does not "adjust itself." You need to give it more
throttle
because there is much less oxygen by volume at ~6200ft. Opening the
throttle not only adds more fuel (via the injectors), but more air...then
it starts. The adjustment needs to be made because at idle, the mixture
is too rich (too much fuel, not enough O2), and the motor stalls (or
won't start). The power loss is due to an overly rich mixture. Airplanes
have a mixture control that is adjusted as they ascend or descend for
exactly the same reason.

--Mike

A modern FI system does "adjust itself" to provide altitude compensation
by use of the MAP sensor input to the computer (MEFI module). Lower
barometric pressure at high altitudes will cause a drop in manifold
pressure, which will be detected by the MAP sensor and cause the computer
to adjust the injector pulse width. Some new FI systems add a fuel
pressure sensor to provide more accurate altitude compensation. Mercury
now uses their own proprietary FI system, but in 1995 it would be a GM
system.

Snowmobilers riding in the mountains have had the altitude compensation
problem for years, until FI came along and solved the problem.