On the bottom of the lower unit near the skag you will find an oil plug,
this is to fill/drain the unit. The two plugs you mentioned up higher: The
lower one is to vent the lower unit while filling/draining, the other is to
drain water from the water pump housing.

I have a feeling that I need to buy a Mercury owner manual or a video
to know the details.

You might want to consider mixing gas and oil at a ratio of one bottle of
Merk oil to 5 gallons of gas. The bottle will tell you to mix one bottle
with 6 gallons of gas. I always mix it heavy on oil, spark plugs are a lot
easier to change than pistons and crankshafts. I have never had any
problems with this heavier mix, Merk oil burns clean and leaves only a
slight residue in the cylinders, usually it can be wiped out with a cloth
during a tear down after 30 years of hard use!

Meks oil injection has not been the most successful thing they ever did and
a lot of motors have burned up because of it. By using gas/oil mix you
eliminate that risk totally.

I didn't know that Mercury oil injection has such a problem. I will
have to research on this issue. Pre-mixing them together is not a bad
idea anyway. The Mercury motor leaks motor oil from somewhere near the
cap of the oil cup when I tilt the motor up (seem like a known problem
from other posts in this newsgroup). I have been thinking of adding
only half a tank of oil to the oil tank to avoid this problem, and
pre-mixing oil will surely avoid this problem altogether.

But this means I will need to get a large portable gas container for
mixing, right? How big a portable gas container should I get for my
boat that has a 44-gal gas tank?

I don't understand your statement "spark plugs are a lot easier to
change than pistons and crankshafts." Do you mean these:

- If we add a lot of oil, the worst thing can happen is having the
spark plug damaged and need to be replaced, and this doesn't cost much,
and can be replaced in the field.

- If we don't add enough oil, the worst thing can happen is having the
pistons and crankshafts damaged and need to be replaced, and this costs
a lot, and is not something that we can replace in the field.

The carbs have a screen filter in the cap where the fuel lines attach. An
external water separating fuel filter between the tank and the motor is a
good idea.

Thanks. I will look around to see if I can find an inline fuel-filter
and retrofit it into the fuel system. The layout of the fuel line and
the available space in the stern doesn't allow me to mount a typical
fuel filter; I will have to get an inline version.

Jay Chan