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…

Practical Sportsbikes (UK) - - Contents - Words: Chris New­big­ging (plus Roberts in­ter­view by Mat Ox­ley) Pic­tures: Jason Critchell

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


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



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

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