Cam Donald Reviews Yamaha TZ750

Date 14.9.2015

Presented by
  • Motorcycle Trader

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Yamaha TZ750

My first bike was a 1976 Suzuki RM 50 and, for an eager five-year-old, it was a fire-breathing monster.

The exhaust note and character of the two-stroke engine filled me with excitement, and its blue smoky waft has left a lasting impression with me ever since.

My road-racing career also began on two-strokes – 125cc GP machines that were perfect for learning about tuning and carburation. From there it was the popular 250cc Production class, comprising the Suzuki RGV and Aprilia RS250 twin-cylinder two-strokes. Sadly, that was the last of two-stroke Production racing in Australia and New Zealand.

During the early 1970s the blue riband GP class was limited to 500cc and dominated by MV Agusta’s four-stroke multi-cylinder machines.

Yamaha TZ750

Yamaha built the YZF500 and it was two versus four-strokes again. It already had many years of two-stroke engineering knowledge from racing in the 125, 250 and 350cc classes and used it to hit the ground running with the YZF500. Winning on debut GP at round one of the 1973 World Championship in France, the YZF500 went on to become one of the greatest GP bikes of all time. During its reign it amassed 115 GP victories, 11 rider’s and nine manufacturer’s titles.

THE TZ750 FIT

At the same time, the US market was prolific for the Japanese brands and Yamaha wanted to promote its products there via racing.

To meet the 750cc AMA rules, a manufacturer was required to build a minimum of 200 examples to enter the bike into racing. Unlike Suzuki or Kawasaki, which had production bikes to build racers from, Yamaha made a big call: it essentially grafted two TZ350 engines together to produce the TZ750.

Though initially a 700cc, this early model successfully scared the pants off its Japanese test riders so Aussie GP legend Kel Carruthers was called upon to help tame the beast. Kel’s advice to lengthen the wheelbase put a stop to the bike’s high-speed tank slapping.

yamaha tz750

As the engine enlarged to 750cc, the chassis adopted the new monoshock rear suspension and triangulated swingarm design.

There wasn’t another off-the-shelf racer that could touch it and Yamaha sold more than 600 of them, winning countless championships along the way. TZ750s won the premier Daytona 200 event every year from 1974 to 1982.

The TZ750 even made a brief appearance in the US dirt track scene when Kenny Roberts entered his TZ750 flat tracker at the 1975 Indy Mile event. It won by beating the dirt-track champ of the time, Jay Springsteen, and a bunch of other riders on Harley-Davidson XR750s.

As the TZ had quite literally smoked the competition with its superior straight-line speed, the AMA quickly changed the rules to ban the machine from further dirt-track competition on “safety grounds” – and to save American-as-apple-pie Harley from total humiliation.

NOT FOR THE FAINT-HEARTED

The way a two-stroke develops its power can be unforgiving, especially prior to advanced electronics to smoothen the power delivery. Power on a multi-cylinder four-stroke is delivered progressively, whereas a two-stroke is more like a light switch – there’s not much until the revs reach the powerband and then, like it or not, you get almost all the power at once!

yamaha tz750

We’re now in a retro muscle-bike boom with Katanas and big inline-fours from the early 1980s being hugely popular, but what about the two-strokes of the same era?
As two-strokes are no longer being produced and raced, their tuning experts are becoming as rare as the bikes. Back in the day, the paddock would be full of men with decades of two-stroke experience.

The biggest task is carburation: to get them running right, carburettor settings need to suit atmospheric conditions. This is a constant work in progress as too rich an air/fuel mixture will have the bike lethargic and reluctant to rev cleanly. Too lean and the bike may seize from too much heat and not enough cylinder lubrication.

One way to check the fuel mixture is to conduct a plug-chop hole, which involves holding the bike at full throttle for several seconds before hitting the kill switch and pulling the clutch simultaneously. You’d then head back in the pits to remove the plugs and inspect their colour where experts would read the colour of the electrode to judge if the fuelling was correct.

Many a two-stroke engine has blown up due to incorrect carburation as a simple error in tuning can have catastrophic effects – both to engine parts and the owner’s wallet!
Phil, the owner of this bike, has fond memories of two-strokes and needed a new classic-racing project, so why not a TZ750, he figured. The thought of another Katana or XR69 production-based bike didn’t appeal like a purebred TZ750, so the decision was simple: build one to go racing.

yamaha-tz750

RARE AS ROCKING HORSE…

This isn’t the kind of bike you walk into a dealer and choose from the showroom stock. You either make a vain attempt to find one for sale (good luck) or you build one yourself.

Phil started off with a chassis kit from CMR in Canada, which produces its version of the original TZ750 chassis with some claimed improvements. Completing the rolling chassis is a set of 17-inch Dymag magnesium racing wheels, comprising a 3.0-inch front and a 5.0-inch rear. Although bigger than original, these rims allow the use of modern race-compound tyres. A conventional 41mm fork from a Yamaha street bike with Ohlins internals and a Works Performance rear shock unit went to Tony from Race Bike Services in Lonsdale, South Australia, who set up the suspension.

The engine was built in Australia from parts and assembled by Barry Ditchburn. Barry knows his way around a two-stroke better than most, having raced and tuned them for much of his life. He was a podium-finisher in GP racing during the late 1970s riding for the factory Kawasaki team alongside Mick Grant and “Kork” Ballington. After relocating from the UK to Australia, Barry won the Australian Sidecar Championship and is a familiar face in the classic racing paddock today.

Cam Donald Yamaha TZ750

Few parts were straight bolt-on items and many needed to be re-engineered or fabricated specifically for the TZ build. Assembling the bike was done by Phil’s mate, Bill White, in Melbourne. Peter McWiggan of Manta Engineering produced much of the hardware and Barry did the engine assembly and tuning, so the bike has some serious classic racing talent behind it.

Since completion in August last year, the bike has been well exercised on the track. Aussie racing legend Malcolm “Wally” Campbell raced the bike at its debut at Phillip Island and again at the Australian Classic Championship in Queensland. It was then raced by Barry’s son, Craig Ditchburn, at the 2015 Phillip Island Classic. At the Island, the bike’s on-board GPS recorded 287km/h.

The bike has been gradually evolving and has gone through many changes. The swingarm pivot height has been changed and the swingarm lengthened for more rear grip and stability.

I’d spoken to Phil about having a spin since I first heard about the project and he kindly offered me a ride – no rider would turn down the opportunity to punt a bike like this.

TAMING THE BEAST

Just sitting on its racestand in the garage, the TZ is imposing – there’s no mistaking this for anything but a thoroughbred racer.

yamaha-tz750

Unlike production racer bikes that show signs of their road-going roots, there’s nothing on a GP bike that isn’t there to make it go faster.

At this year’s Penrite Broadford Bike Bonanza in Victoria I finally had the chance to throw a leg over the TZ.

Mounted on its racestand, I waited for the remote roller to spin the back wheel to start the bike. Unlike the classic inline four-strokes I often ride, the TZ didn’t need much momentum to fire.

The multi-cylinder 750cc two-stroke gurgled to life and a couple of healthy blips of the throttle had the engine revving cleanly. The exhaust note, to a two-stroke lover, is simply music: a sweet-sounding combination of raspy crack from the exhausts with the intake growl of the four carburettors, all without an airbox or any air filters.
The smell of a blue cloud of Elf HTX GP oil and Avgas is another hit to excite the senses. It also takes me back to my childhood with my RM 50.

Out on track, I opened the throttle and the tacho needle shot towards 10,000rpm. I shifted up a gear and the needle did it again, another gear showing how quickly the engine makes power. Maximum engine speed is 10,620rpm. A two-stroke race bike is either revving hard and asking for a higher gear, or it’s ‘off the pipe’ and searching for a lower one, and there isn’t a lot in between.

yamaha-tz750

The bike feels small – even compared with a compact, modern sportsbike – and it feels light. Tipping the scales at just 145 kilograms, the bike is a featherweight for the performance on offer. On a dyno, the liquid-cooled inline four produces an impressive 164hp (122kW) at 10,300rpm and 117Nm of torque.

A lack of weight and bulk allows a bike to perform better, from acceleration to braking. And combined with the lack of internal inertia, there’s a big response from little effort in the steering.

The TZ is a pleasure to ride, and an exciting one. Gear selection is crucial to keep power in its sweet spot.

I tried to keep the bike driving through the middle of turns and not hit its powerband until I’d picked the bike relatively upright on the tyre.

On a dry track, sunny day and the latest Dunlop Superbike tyres, it was all laughs and giggles. Trying to imagine a similar bike in the late 1970s with skinny tyres and ordinary grip at best, combined with a slippery surface or wet conditions doesn’t bear thinking about.

yamaha tz750

As fun as the TZ is to ride, it demands respect and I gave it plenty. There was no stopwatch or race result to care about and I simply enjoyed each lap while building confidence and speed. Man, these bikes are demanding to ride hard.

After a solid track session I returned to the pit to digest the experience.

I was still smiling at how strong the engine was, but felt the bike was under-sprung. Swapping to heavier-rate springs in the fork and shock would take care of this and control the excessive suspension movement.

On paper, the TZ should be a match for any classic bike on the grid, but results could tell a different story. With current classic events usually held over six- or eight-lap sprint races, the slower-starting two-stroke is at a disadvantage in the drag race to the first turn.

Engine characteristics make it difficult for the two-stroke to pass.

A four-stroke will slow into a corner, turn and then jump off the exit. A two-stroke will carry much higher rolling speed through the whole turn, whereas four-strokes have a wider power curve and usually more torque to be stronger off the corner. If the two-stroke gets a clear track, it may be able to clear off, but if stuck in a battle with four-strokes spoiling its mid-turn strengths, it can be very difficult to ride to its full potential.

Given this is a relatively new build, it’s well on the way to being a fully sorted race bike. The owner plans to race it again soon with a high-profile name from Australia’s recent racing history. Watch this space.