T
he original M3.5B
was released on
the Oz market
in 1991 and
since then little
has changed. Based on
the direct-drive M3.5A
introduced in 1975 it has a
clutch and a deeper 2.15:1
gear reduction. The 50:1
engine has only one piston
ring and ignition timing
fixed at 20 degrees Before
Top Dead Centre. A 1.4lt
integral tank gravity feeds
to single-jet carbie and the
throttle is a simple bar lever
next to the choke lever.
Maintaining the M3.5B is
a pain because the clamshell
cowling needs to be carefully
screwed together. A separate
hatch accesses the spark
plug. Servicing intervals are
every 50 hours or annually
after the first service at 20
hours. The break-in period is
five hours on 25:1.
Because of the fixed
ignition timing the engine
runs too advanced at low
rpm and has fairly high
vibration levels. It’s best run
as much as possible at about
two-thirds throttle opening
where the fixed timing gives
maximum fuel efficiency.
The MFS 3.5B was releasedseatrials
Single M3.5B and MFS 3.5B on 1978 flat-bottomed 3m deHavilland John 10 punt, pushing a
total of 220kg and 225kg and swinging 7-inch and 5.7-inch plastic weedless props
respectively. Average of two-way runs on Lake Macquarie, NSW, in calm water. Range is
in nautical miles with a 10% reserve.
ENGINE M3.5B MFS 3.5B
TROLL RPM 1200 1500
KTS 1.8 2.4
LT/H 0.2 0.1
RANGE 11 21
2/3 THROTTLE 4100rpm 4500rpm
KTS 5.9 5.8
LT/H 1.1 0.8
RANGE 6 7
WOT RPM 5200 5700
KTS 6.6 6.5
LT/H 1.6 1.1
RANGE 5 6
“Loop” of cruising with 10% WOT and 40% trolling operation, averaging 4kts
LT/H 0.73 0.57
RANGE 6 6Tohatsu M3.5B
versus MFS 3.5B
SIMPLICITY TO SMALL MOTOR SOPHISTICATION...
in 2006 as the MFS 3.5A and
was Tohatsu’s first four-
stroke under 5bhp. It also
has a clutch and the same
reduction ratio as the M3.5B
but develops its maximum
output much higher up
the rpm range. However it
has a twin-jet carbie and
electronic timing advance
from three to 25 degrees
BTDC so it’s a much sweeter-
running engine. The single-
cylinder OHV engine has
gear-driven push rods andsplash lubrication as there’s
no shallow-water drive
setting to reduce lubricating
efficiency. A thermostatically
controlled cooling system
helps the MFS 3.5B gain
an OEDA 3-star emissions
rating whereas the M3.5B
has a “0” rating.
Because the 1lt fuel tank
is mounted atop the engine,
access to the rocker cover for
checking valve clearances is
limited. At least the cowl lifts
off the engine pan for carbieand spark plug access and a
neat little elbow under the
engine makes draining the
0.3lt sump easy. A twist-grip
throttle and pull-out choke
knob make operating the
MFS 3.5B easier, but like the
M3.5B there are four trim
positions and a stopper knob
for full leg tilt. Both engines
are designed for 16-inch
transom heights.
The recreational usage
warranty is three years for
both engines.30 tradeaboat.com.au