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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.
Picture: Kerry Wilton’s TZ750B
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.
Picture: TZ750A ( Courtesy Joris van de Wiele )
Technically, though very similar to the TZ350 motor-wise, it differed in a few crucial areas, these being:
1. 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.
2. 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.
3. The 64mm dia. pistons had inlet holes cast into their rear, though a few of the early examples did not have this.
4. 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. (Click here for information on new TZ750 piston kits for sale) 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.)
( Picture: OW31 cylinder, courtesy of YamahaRick )
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 Ds 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 Ds.
The Ds sold for Ј7,000 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.)
Picture: TZ750F factory shot (Courtesy John Hulme)
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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