Sounds like you have a fuel starvation problem.
You can do some diagnosis if you take an extra person to the lake w/ you to
avoid buying parts as a trail and error.
First - remove the pickup from the tank and make sure the screen is clean on
the bottom of the pickup.
Second - remove the fuel filter and make sure it is good an clean.
Then go to the lake and run.
When the problem happens have the extra person check the fuel line bubble to
make sure it is not collapsed.
If it is - replace the fuel line bubble.
If not try pumping the bubble and see if the problem goes away. If so -
then the fuel pump needs replacing.
If not - try engaging the choke and see if the engine picks up speed. The
choke is a fuel enricher and not an actual butterfly. If it picks up you
have a carb problem.
If not or boggs more then you probably have an intermittent ignition problem
from a stator or powerpack.


--
Tony
my boats and cars at http://t.thomas.home.mchsi.com

-
"Grant" wrote in message
ups.com...
Hello,

I bought a used Whaler with a 2004 70 HP Yamaha outboard. I used it in
last fall and again this spring on local lakes. It ran fine.

It recently developed a problem which I'll describe. I'm looking for
advice as to where the problem might be.

I took it out yesterday and ran it at around 14K RPM out of the local
harbor. I then accelerated up to around 40K RPM and it did fine. I
slowed back down to around 14K RPM for around 10 minutes and then when
I tried to accelerate the RPMs actually decreased as I tried to give it
more gas to the point where the engine would stall if I gave it too
much. If I backed off the accelerator it would return to around 14K
RPM.

It I shifted into neutral and accelerated I could run it up to 40K RPM
without problem. However, doing the same when in gear would cause the
problem.

I played with it for a while as I slowly motored back to port and
noticed that the RPMs seemed to be increasing. Sure enough, easing the
throttle up it went right up without any stall. I zoomed back to the
harbor and slowed down again as I entered and then as a test tried to
accelerate again and sure enough the problem was back.

So, someone suggested that I check my fuel filter to see if I had water
in it but it doesn't appear that I do. I had the whole fuel line
flushed and the external filter was replaced as part of that fix up
about 3 weeks ago. I had used the boat at high speed 3 or 4 times
since without problems.

Thanks!

Grant Schenck