44
A classic
kicker
Cruising Helmsman July 2018THIS LITTLE OUTBOARD WAS A HIT WITH CRUISERS GIVEN
ITS WIDE THROTTLE RANGE AND CHOICE OF PROPS.
PRACTICAL
ENGINES
RELEASED on the Aussie market in
1988 and discontinued in 2006, the
M2.5A was a derated version of the M3.5A
originally released in 1975. It was a simple
direct-drive, two stroke that suited small
inf latables and dinghies and racked up an
impressive reputation for reliability.
The WT47B water cooled, two stroke,
47 millimetre cylinder bore, B series power
head was also used for: the Mercury 2.2
released in 1985; the Mariner/Mercury
M2.5 in 1990 and the Force 3 in 1991.
All of these were direct drive outboards.
All variants of the WT47B ran on a
50:1 premix. Originally the break-in mix
during the first ten hours was 20:1 using
dedicated outboard oil but, if this was
not available, a 15:1 mix of unleaded and
SAE30 four stroke lawnmower oil could be
used, leaning to 20:1 after break-in. In later
models the break-in time was reduced
to five hours on 25:1 of unleaded and
outboard oil, preferably Valvoline TCW-3.
Why the M2.5A stood out from all of
these was its massive wide open throttle
(WOT) range of 1400 revolutions per
minute, ensuring the engine was never
likely to be overloaded. Combined with a
choice of six props from 4.3 inches to seveninches of pitch, the M2.5A suited more
hulls than the direct competition.
As with all of the variants, the M2.5A was
designed for transom heights to 16" whereas
the Suzuki and Yamaha competition were
intended for transoms no more than 15".
If either of these are used on a 16" transom
they suffer cooling water starvation due
to aerated water from under the transom.JUST THE FACTS
The WT47B power head is the essence
of simplicity, making it ideal for cruising
yachties who may not have access to
service facilities. For example, there was
just one piston ring that did the duties of
both maintaining cylinder compression
and scraping oil.
The carburettor had a single jet with
a needle valve that could be raised or
lowered in four steps to make the air/fuel
ratio richer or leaner. Because the M2.5A
developed only 69 per cent of the M3.5A’s
output the needle was set lean, resulting
in fuel efficiency not far behind small four
stroke outboards.
The carbie was gravity fed from a 1.4
litre integral fuel tank via a fuel cock.The throttle and choke slide controls
were connected directly to the carbie
eliminating cables and linkages.
Up until 1991 all variants of the WT47B
power head had a Mitsubishi Electric
f lywheel magneto with breaker points and
the option of a 12 volt 40 watt alternator, but
these were dropped in favour of CD ignition.
Even with breaker points the ignition
system was strong enough to virtually
eliminate spark plug fouling, whereas
the direct competition had much smaller
electrode gaps and easily fouled their plugs
during extended low speed operation.
The M2.5A, however, has retained its fixed
ignition timing of 20 degrees before top
dead centre (BTDC) compared to normal
ignition advance of around five degrees
BTDC when idling up to around 25° BTDC
at WOT. This means the M2.5A runs overly
advanced at low to midrange rpm resulting
in much higher vibration levels than
comparable output four stroke outboards.
At least the M2.5A has a f lat torque curve,
with 3.9 Newton metres available at a low
3000rpm, peaking at 4.1N.m at 3700rpm.
All of the direct drive variants cleverly
incorporated the water pump impeller in
the gear case, a design feature adopted byANDREW NORTON