Re: [OMC-Boats] Engine RPM and Boat Speed

On Tue, 29 Jul 2008, Lee Shuster wrote:
> So without reviewing all of your data points again, this sounds less
> like "slippage" and more like a case of a combination of too little
> torque for the combination of altitude, weight, and prop pitch.
> Just my, shoot from the hip, two cents worth..

I've got the same 155 hp drive with a 14x18 prop in a 16' boat and have no
trouble getting to 30+ mph, even with a waterlogged hull. I'm not good
with diagnosing problems with older engines, but I'd start out by replacing
the fuel filters. It's likely you have several - one on the hose coming
from the tank and one on the inlet to the carb. My boat has the inlet
filter removed and I only use the inline one on the hose - if your inlet
filter is still installed it's likely pretty clogged. Clogged filters
restrict fuel flow to engine and can limit max power (plus make it run
lean, which isn't good for the engine either).

Replacing the spark plugs and wires might be helpful as well, though I've
only ever had problems with starts or rough running due to those. While
you have the plugs out, you can do a compression test on each cylinder (a
tool costs $25, or some places loan them out) to make sure your rings and
valves are still good.

Make sure the throttle linkage is opening fully - sometimes they slip out
of adjustment and that can limit your power. To check this, pull the spark
arrestor cover off the carb and make sure the butterfly opens fully.

The next step is likely a "tune-up" involving adjusting the carb - I
haven't learned how to do that yet.

As long as you're checking all of this, try tilting the drive all the way
up (so the outdrive and engine ball gears disengage), putting it in gear
(try both forward and reverse), and rotating the prop by hand. The entire
geartrain should rotate freely with no binding. Of course this should all
be done with the engine not running, and try to minimize the time it is in
gear while out of the water, as the shifting coils are supposedly water
cooled and can overheat when run in air. It's possible you can't tilt it
up far enough to disengage it - I can't remember if I did this test with
the drive installed or removed.

Ethan

-- 
<a href="http://www.engr.wisc.edu/~brodskye/"> Ethan Brodsky </a>