High on the pipe-dream list for many two-stroke fans is a Roberts-rep TZ750. Brian Spencer has made the myth a reality: and it all started with a set Harley forks won on ebay…
The original badass dirt bike faithfully replicated as a sprinter. Kenny would approve
Every cloud has a silver lining – apparently. But for Brian Spencer, his cloud became lined with another cloud: a big, blue smokey cloud. In 2015, he was knocked off his Mvagusta Brutale 800, breaking his hand and collarbone, and had a ten-month stint off work as surgery and healing did their thing. If you’ve ever had the misfortune of such a recuperation period, you’ll know how dull it gets – and fast.
“I had to find something to do. So I spent a lot of time looking on the internet – searching ebay, mostly…”
One of Brian’s pre-crash interests was fettling and racing hisaermacchi 350. “Mostly at local club meets,” he says. “And I’d let someone race it at the Manx most years too.” It was searching for listings of Ceriani forks for the ’macchi that Brian came across similar sets – and that started the TZ750 project.
“I bought a set of Ceriani replicas, made by Forcella Italia who bought the rights to the company.they’re 42mm instead of 35mm like myaermacchi, but they look the same. They came with the yokes from anamerican company called Storz, who sell flat-track conversion kits for Sportsters. I decided they needed to be in a bike like that, and came up with the idea of ATZ750 ’tracker like Kenny Roberts’s.
Brian began hunting for a completetz750 to begin with, though they’re neither common nor cheap.through his classic racing connections, he spoke to Steveabbott – 2002 world sidecar champ, and something of a specialist inyamaha straight-four racing ’stroker engines – in chairs primarily, but he’ll give solo powerplants an eye over too. He hit paydirt: Steve had an engine.
“He was really helpful, and sold me a complete engine. Obviously I didn’t have a bike, so I needed the carbs, electrics – everything, and he got it for me. Even parts like the throttle – it’s a one-into-four cable arrangement which isn’t easy to get.”
Two major components down, Brian was obviously still missing the key element: a frame. Calls to the usual suspects like Harris Performance came up fruitless – nobody has a jig at all for ATZ motor, let alone one to built a flat-track-inspired bike.the search led him out of the UK, though to another Brit across the water.
“Denis Curtis is an ex-pat from Leicestershire, and he’s been in Canada since the late 1960s building chassis. CMR Racing have experience in flat track frames, but also build road-race chassis kits for thetz750. He
“BRIAN WAS STILL MISSING THE KEY ELEMENT: A FRAME. A SEARCH LED HIM TO ANOTHER BRIT ACROSS THE WATER”
was another really helpful guy – we talked about what I wanted to use the bike for, and he tweaked the geometry to suit what I wanted to do.
“I’m not interested in using it for dirt track, though I wanted the 19-inch wheels for the look. I intended to use it for sprints and hill climbing, so he built it with sharper geometry and more weight over the front, more like a road racer.”
CMR proved another great choice for the build – the kit supplied included a tank, swingarm, footrests, a pair of Öhlins shocks and every lug on the frame ready to accept parts and bolt together.
“I made a mistake ordering the rads though. NEWTZ rads are £1600, repros are still £850… and a Chinese radiator was less than £100. So I ordered one, and told Denis what I’d bought: but he warned me the wheels and geometry I chose wouldn’t leave enough room for an original radiator. So he suggested a curved radiator from a 2004-06 Yamaha R1 – and a Chinese pattern part was only £86.”
The pipes are Swarbrick: made in cone kit form in Preston, shipped to CMR in British Columbia, welded in Newyork State in the USA, back to CMR, then eventually to Brian, who lives just a few miles from Swarbrick in the northwest of the UK.
Kenny’s Indy Mile bike used flat-sided expansions, all tucked under the motor with the prism-like chambers neatly fitted together and exiting low either side of the Goodyear dirt tyre. But they were prone to splitting: so round-section road race chambers it is.
When the kit arrived in mid-2016, it was the special builder’s dream – everything just bolted up. Brian enlisted the help of off-road/ supermoto guru Dave Clarke and his network of contacts to work through the details.andy Fawcett machined the wheel spindles, a new stem to suit the Storz yokes/ CMR frame combination, and he milled the chunky, superfluous mudguard mounts from the fork bottoms. He also made a small modification to one of the expansion chambers to make it sit tighter.
The wheels are built usingtz350 hubs and 19-inch Excel rims, built by Markwillis.the six-piston PFM calipers and twin 320mm discs are obviously not in keeping with the original, but are in keeping with selfpreservation. “It wasn’t intended to be accurate, but a tribute that I can use. I’ve never understood building bikes purely for show, so it was important that the bike could be ridden, too.”the rear brake is an opposed-piston Brembo, supplied by CMR.
More idle time and ebay turned up a Spondon disc with a magnesium centre, which has been stripped, checked for cracks and refinished.
At this point, Steveabbott called Brian with an offer of another engine…
“Whentz750s let go, they damage crankcases, so they all have some sort of repair.the engine I built the bike around had a lot of repairs, but in the meantime, Steve found me one in much better condition, so I part-exed the original for the one that’s in the bike now.”
The bike was stripped after the dry build and then treated to powdercoat where necessary and paint by Dave Higginson, before a final build. It made its first proper outing at the Leighton Hall hill climb last year: Brian’s hand still isn’t fully recovered, so Dave Clarke selflessly stepped in to run it.
“It was having a problem jumping out of first gear, so Dave went easy on it. But he set reasonable times considering it was its first real run anywhere.”
The problem proved to be an incorrectly installed gear selector fork, which Steve Abbott corrected easily. Now it’s ready for
“I’M NOT INTERESTED IN DIRT TRACK, IT’S BUILT WITH SHARPER GEOMETRY LIKE A ROAD RACER”
It fires with the start of electric rollers with ease and intimidation.a tweak on the throttle is all it takes for the motor to catch and the four expansions to bark their noxious contents in to the atmosphere, as the four 36mm Mikunis gobble up more of it. It’s crisp, free-revving and oozing intent. Excited?yes. Intimidated?also yes.
It takes a while to get the Scitsu temperature gauge up to the 55-60ºc optimum, and an exploratory first lap (about three miles) sees it drop like a stone, so it’s back in for a strip or two of tape on the radiator, and back out, this time with a bit more gas.
Gently away with minimal clutch slip in first to preserve the dry plates, then wind it on. Power comes in at seven grand – I think, I’m not really watching the tacho as the experience unfolds – and the short-gearedtz leaps up like there’s an electromagnet tugging at the ferrous elements in the front end. Let it drop, rev out, hit second, wheelies again, drop it, select third, wheelie again. It goes again in fourth until I actively get a handle on it.
Brunters has a pretty haggard surface – all joins, potholes and other surface imperfections waiting for a chassis to upset. Brian has a steering damper kit awaiting a fork clamp at home, and on this, the bike’s first proper run into the upper gears, I can confidently say it needs it.the sharp head angle, short wheelbase, minimal weight and hard-hitting power easily unload the front tyre and put it on the brink of a weave or slap.tucking in helps a little bit, reducing the human sail effect. I’m even briefly tempted to move my left hand from ’bar to fork leg, Indy Mile style, but there’s no way I’m risking the reduction in control…
At last, corners appear, and the PFM brakes calm the situation. It steers well but the forks/ shock combo isn’t well matched. Maxton will be seeing some of the Spencer income soon to correct it. So no heroics from me today: the Maxxis dirt track tyres lack support to really attack or put the power on leant over.
Who cares?the corners open out, thetz is back on the pipe, waving it’s front wheel in the air, and snaking its way along the runway return. It’s not to be trusted for second – it’s everything you might assume, and that’s only on a brief, respectful test on tarmac.
I can’t conceive of what it was like in the white heat of competition at 150mph, with no brakes, no run-off and loose dirt to deal with. Brian’s tarmac-going tribute gives a terrifying insight in to what he was dealing with: truly one of the wildest combinations of frame and engine ever conceived.
There’s really nowhere else for number one cylinder’s spannie to go – so behind and up it is then
4 Öhlins yes, but not everything from the Big Ö makes for perfect handling. The suspension needs some work – and Brian knows that 4
3 Throttle arrangement is a singlecable into a four way junction. Action is light and quick enough too 3
2 2 That’s a Spondon disc clamped by a floating Brembo caliper to promote some squat under braking
1 Rear frame loop squared off with shaved corners to follow lines of bespoke tail unit 1
Filterless round-slide 36mm Mikunis meter the premix