Sounds like you have too big a prop. Probably the original owner did not
carry many people and wanted speed at a lower throttle setting.
3400 rpms at 3/4 throttle is low.

1. Do a compression test to verify no problems w/ the engine.
2. Install fresh plugs. While they are out look at the old ones and verify
each cylinder was firing.
3. Check the prop. Does this boat have dual props?
Call Doral and find out what prop the boat should have on it.
4. If these all fail - the carb may need a rebuild.

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

-
wrote in message
oups.com...
My wife and I bought a 2000 Doral 250se with a Mercrusier 454. This
boat has only 67 hours and has not been used in two years. The boat has
been winterized by a local marina. This is the only service that's been
done to it. When driving it to another marina, it seemed a little
sluggish. We were able to plane in about 20 sec. and reach a speed of
42 mph at 3/4 throttle.(3400 rpm) However, with 5 people on board, we
could not plane at all. The best we could do was about 8 knots. We
tried trim tabs and looked for weeds or obstruction on the outdrive.
The engine seems to run very smooth in gear, at idle, up to about 1400
rpms. When 3/4 throttle was applied, the engine seemed to work hard.
(about 3200 rpm). Full throttle didn't change a thing. What are the
possible concerns or problems?

Thank you for any help, Ron