Yamaha
TZ750
TZ250
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Right from the conception of Yamaha's all conquering
TZ350 and 250 the factory was already developing
a 700cc, 4 cylinder "doubled up" version
of the 350cc twin. By casting special, wider
engine cases, to allow the fitment of what was,
essentially ( though not exactly ), a pair of
350 top ends, they had created an in-line 4
cylinder 2 stroke production racer engine, in
the bigger capacity.
 Despite
releasing nine different models of factory 500cc
GP race bikes for the contracted top level riders
to use, it wasn't until the end of the decade
that Yamaha released their production 500cc
GP racer. Unlike the 750, this bike was a little
more unique when compared to the smaller capacity
twins.
The mighty TZ 750
In 1972 Suzuki stunned the world at the Daytona
200 by turning up with a brace of watercooled,
750cc triple cylinder racers producing an incredible
(for the early 70's) 100bhp. Unfortunately for
them, all of the bikes DNF'd due mainly to the
power destroying their rear tyres.
The cards were on the table, Yamaha knew they
had to do something to counter this threat and
also that of Kawasaki's immensely fast, yet
at times un-reliable KR750, if it was to have
any chance in the new Formula 750 class. A prototype
was constructed and Kel Carruthers tested it,
coming away believing the approximate 90bhp
it was producing was "lazy" and the
bike was capable of a lot more. He was right.
Yamaha unleashed it's first production 4 cylinder
750 two stroke racer monster on the public in
March 1974, in the shape of the awesome TZ750A.
Priced at around $Aus3,500 this bike had in
fact been under development as early as 1971.
This ground-breaking model weighed in at 157kg
dry and produced 90bhp @ 10,500rpm from it's
watercooled 694cc engine. Formidable figures
in the early seventies by any standard. Interestingly,
Yamaha claimed the bike had the potential to
produce almost 140bhp with TZ350 cylinders fitted.
Technically, though very similar to the TZ350
motor-wise, it differed in a few crucial areas,
these being:
- The head's squish band was reduced from
the 350's 2.0mm to 1mm and it's combustion
chamber was made a little deeper so as to
keep the compression ratio to 7.3:1.
- The exhaust port was lowered 1.5mm and
four petal reed valves added to help control
the influx of fuel mixture from the 34mm Mikuni
carbs and to help "tame" the power
delivery of this awesome machine , along with
an additional fifth transfer port, "inlet"
port if you like.
- The 64mm dia. pistons had inlet holes cast
into their rear, though a few of the early
examples did not have this.
- The four cylinder firing order was 1 and
4 (simultaneously) then 2 and 3.
Due to the difficulty the factory had trying
to fit the four huge expansion chambers underneath
the bike they chose to make the belly section
of each basically box shaped to utilise the
limited space available. Unfortunately the shape
caused the pipes to be prone to splitting open,
a problem rectified by owners and tuners by
simply cutting each pipe open and welding short
pieces of spoke wire across in an "X"
pattern as a reinforcement measure. Other problems
such as cylinder head nuts splitting causing
water leaks and main bearings seizing appeared
at times as well.
 From
the "A" model the TZ750 underwent
minor improvements for the following year's
"B". There were just five; an up-rated
waterpump to better handle the cooling duties
for the four screaming cylinders, strengthening
of the split-prone chambers, a "beefed
up" chain tensioner, a couple of gears
were improved and most importantly, an increase
in bore diameter to 66.4mm to take the formerly
694cc machine out to a full 747cc, though the
first 46 "B's" were produced with
the smaller capacity, only after these bikes
rolled off the production line did the bigger
engines appear. The improved chain tensioner
was very welcome because one component that
really copped a beating on a TZ750 was the drive
chain. Savvy owners eventually worked out that
the chain needed to be pre-stretched in order
to last the distance in a longer event !
The "C" was unchanged from the "B"
and was really just a way the factory could
supply the big machines to those in need while
the "D" was being developed and produced.
The OW31 works racer was released around this
time. Motor-wise the bike had 6 transfer ports
per cylinder, unlike the stock TZ750's. Other
improvements over the customer 750's were copious
amounts of titanium and magnesium to save 18kg
in weight and a mono-shock frame. (Picture:
OW31 factory poster shot supplied by Tim Keyes.)
1977's TZ750 "D" was marketed by
Yamaha as a "works" OW31 replica and
as a result a high percentage of (though not
all) owners like to claim their "D"s
are OW31's, when in fact they are little more
than a mono-shock "C" with mufflers.
(Still an absolutely awesome machine none-the-less.)
None of the exotic metals or components from
the OW31 were used on the "D", obviously
to keep costs down. The only changes to the
motor were alterations to the pistons, exhaust
ports, jetting, crankshafts and ignition wiring.
Other components to receive an upgrade were
the exhausts, which now had the left hand outer
pipe twisting around behind the carbs to allow
the chambers underneath the motor to be the
correct round section. The exhausts were also
now fitted with silencers, and the frame bracing
was increased. Only about 20 or so genuine OW31's
were produced that year, and just 10 more (30)
TZ750 "D"s. The "D"s sold
for £7,000 including a spares kit.
Over the next two years, 1978 and 1979, 162
more of the "OW31 replica" TZ750's
were made. Unfortunately, the bikes remained
basically unchanged from the "D" model,
apart from 6 petal reed valves being introduced,
though with an output of 120bhp @ 11,000rpm
utilising a full 747cc and pushing just 152kg
dry, they were not to be ignored. ( "They"
say the roofs of the transfers had their angles
changed with the "F", though this
is unconfirmed.)
Sadly, the FIM dropped the Formula 750 class
from World Championship status in 1979, effectively
ending the TZ750's reign.
Apart from competitively running in various
Formula One classes throughout the world until
1983, and of course, today's "Forgotten
Era" class racing, the "beast"
has become little more than a fond memory of
those who raced, worked on, and observed this
incredible machine. Not forgetting of course,
a truly prized possession of the lucky few who
still own one.
The last TZ750 "F" was sold in January
1983.
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