The problem is the pump is not producing enough pressure to raise the engine
under load.
Probably not something you can repair where you are.
Either the pump is bad or the o-ring in the piston is not holding pressure.

Try this - raise the engine up about 1/2 way and then run the engine - does
the engine drop under load. If it does - you have a seal leak in the
piston. If not - it is a pump problem.
Verify you have the right amount of fluid in the system (not sure on a
Yamaha how to check this). Otherwise - you have a pump rebuild/replace
ahead of you.
--
Tony
my boats at http://t.thomas.home.mchsi.com

"RichG" wrote in message
m...
Strange, but here's the story.

I have a 40 hp '97 Yamaha The tilt trim works GREAT when the motor is at
rest, ...up and down all day long. It works GREAT when the key is on and
the boat is in the water ...up and down all day long...It holds its raised
position if I leave the motor raised up all night long!!!

but.....

When the boat is under way, even moving very slowly, it will go down, but
it won't go up . It tries. I can hear and see the hydraulic shaft trying
to
raise the engine. It doesn't seem to have enough power, even at very low
speeds???

My mechanic, ( by phone, from 1200 miles away ) suggested I try

1. bleeding the system ( I did..to no avail) and
2. check the "bypass".... (I don't know where the "bypass " is).... to be
certain that someone didn't leave it loose.. He couldn't sufficiently
describe it's location over the phone. I did find an Allen wrench (
metric)
fitting in the cylinder area, but it was very tight. The motor is mounted
on a manual jack plate, so it is possible that I cannot see the bypass
without pulling the engine from the JP. ??? Anyone know where the d%%#$#
thing is???

I don't want to bring it to an unfamiliar dealer here ( Lake Geneva,
Wisconsin) since I know they are all backed up and I don't want to be
with-out my boat for three weeks right now.

Any ideas? If I had to guess, I'd think that the voltage to the t/t
motor
is too low... when under way. I checked (looked at ) the connections, and
they look great inside of the engine. I did NOT take any voltage
measurements, but will if some one thinks that might tell me something
useful.

Any help or advice appreciated. Thanks RichG

-- manager, Carolina Skiff Owners Group on MSN
http://groups.msn.com/CarolinaSkiffOwners