TZ750 STREETFIGHTER

It's Alive! Dr. Sean Dwyer transforms ex-works TZ750 into a one-off monster.

story by David C. Walsh , Photograph by David C. Walsh , Created Jul. 23, 2004
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Bethesda, Maryland ~ Dr. Sean Dwyer has raced BSAs, big Wood-Rotax singles and Yamaha TZ350s. He currently street rides an R90S Beemer, and a Vincent Black Knight.

In the remaining interstices of life, he's managed to rebuild one of the all time great works-based specials.

It's early fall, a lazy Sunday in a wooded neighborhood in the suburbs of Washington, DC. Suddenly, the peace is rent by an otherworldly shriek. Moments before, cardiologist Sean Dwyer had broomed aside marble-like acorns from his sloping driveway and bumped started his TZ750 Yamaha, an ex-"works" bike turned "naked" special. He's blipping the throttle gently because even indulgent neighbors have their limits! The jaunt to follow will continue the bike's running-in process. Only not here.

At a turnout beside a road less traveled, we roll the bike down the ramp and Dwyer mixes the two-stroke's special 16:1 gas-oil elixir. I push him off and with a sound totally unrelated to an old RD 350's he's off the shady turnout and gone.

The normally dour doc pulls up all smiles and grins. Had the new "friendlier" gearing helped? Indeed! "Didja' see the front wheel come up? The front was shaking & I think I need a steering damper!"After a period spent blitzing traffic up and down the road like a man possessed, the normally dour doc pulls up all smiles and grins. Had the new "friendlier" gearing helped? Indeed! "Didja' see the front wheel come up? The front was shaking & I think I need a steering damper!" We managed a few snaps in our time in public, but were almost shut down by a stone-faced US Park Policeman. "Visitors want peace, not smoke and excitement." Sure, but the tree pruners with their chain saws were just as loud and.... (Shut up, Walsh!) By a hair's breadth we avoided an impounded bike - or worse.

"I'd raced TZ350s for a number of years," Dwyer goes on, "so I sorta' had the 'TZ Disease.' "

Back from our "action shoot," we give the still-toasty four-cylinder racer a once-over. It's tiny, save for the giant engine in the Spondon frame. With few nods to fashion or legality (pip-squeak horn, prop stand, simple lighting), the racetrack pedigree is obvious to any aficionado of Yamaha's late '70s 'strokers. This whole project began, Dwyer relates, when friend and local TZ wizard Gary Bernstein mentioned a race bike going to seed down in Florida. The owner "decided he was keener on jet-skis than he was on reconstituting an exotic old two-wheeler." His loss. "I'd raced TZ350s for a number of years," Dwyer goes on, "so I sorta' had the 'TZ Disease.' " That forgotten Florida project bike was comprised of a disassembled motor, a great big bucket o' bolts and one complete Spondon race frame. It was pronounced "good" by another Dwyer mate in Georgia, but cobbling it all together would entail more than a quick stitch job. Clamp! Sutures! Circlip pliers!

Happily, a large selection of spares was included with the neglected bike. Those spares proved the dealmaDo you get the feeling Dr. Dwyer is thinking "Ok, so what's next"? ker, as Dwyer the TZ350 campaigner knew parts for these pure race bikes were achingly expensive and exceedingly rare. Ultimately, the mélange was trucked from Florida to Maryland, by vintage Triumph racer John Gallivan.

Like most things worth doing, the rebuild took time and grit. "I was fooling around with it," Dwyer recalls, "between a ton of other things, racing a lot of bikes and go-karts - but I sort of pressed on." Like any good doctor, he consulted specialists here and abroad, ending up with a complete 1990 Suzuki GSXR750 front end and narrow-rimmed 1980s rear wheel, both from salvage yards. For the front, I machined up the various carriers for the bearings 'n stuff, to fit that. Then, I got an early Gixxer rear wheel with the correct 4 1/2" rim." (The first one was too wide at 5 1/2 inches).  "King Kenny" also flat-tracked a TZ700 very similar to this 750, afterwards remarking, "They can't pay me enough to ride that thing!" "I machined the sprocket carrier, narrowed it down, sunk the bearing in and moved the sprocket in to fit inside the Spondon swingarm." After much machine work, he'd grafted the Gixxer 750 wheel, rotor and caliper onto the swingarm. "I narrowed the rear caliper, fabricated a caliper hanger and squeezed the package into the swing arm with only shim room to spare."

Yamaha produced some 300 of the big 700 & 750cc TZs in their 6-year production life, including a half-dozen variants. Some were superbike-class machines geared for "horsepower" tracks like Daytona, others for tighter European roadrace circuits, with a couple even configured as dirt-trackers. The best-known TZ pilot was Kenny Roberts. He won the big bike-dominated 19__ Daytona 200 miler on a TZ350! "King Kenny" also flat-tracked a TZ700 very similar to this 750, afterwards remarking, "They can't pay me enough to ride that thing!" The remark, along with the bike's smoky fumes, helped sound the death knell to the screaming racer and in fact all two-strokes in the 'States.

He'd hoped to get an original TZ750 front end and rear wheel, but "They were incredibly pricey and not worth it you know, when I could find a setup that was much better for a street special." (Dwyer's a bit ho-hum about this work, but as is typical with specials, the entire process was maddeningly slow with countless cut fingers and binned "attempts.") The Suzuki's stanchions are several mil larger than the works items, and their four-piston brakes are a big improvement on the stock racer's two-pot items. Another key asset was the tubular Spondon frame, which mated with unexpected ease to the more robust Suzuki pieces. Dwyer explains that the "stiffer and stouter" British-made race frames largely did away with the flex problems of the factory iterations. (In the early 1970s, racers Kel Carruthers and Kenny Roberts commented rudely on the big TeeZee's handling). Page 2   "I was fooling around with it, between a ton of other things, racing a lot of bikes and go-karts - but I sort of pressed on."

When it comes to the motor, Dwyer says, "I got it together with some fiddling around." It's thought to be a factory replacement, "because the eight-digit serial number on the engine case was blank except for the first three TZ700/750 designator numbers." Dwyer reckons the motor once powered a sidecar rig in England, because of the brass vacuum plug emerging from the crankcase. "Those were used by the racing sidecar guys over there to drive a fuel pump." The internals were clean, complete and "pretty serviceable" - besides proving remarkably easy to work on for a race machine. The high-performance TZs may have shredded the era's skinny tires with a vengeance, but their trick two-piece crankshaft and other innards appear bulletproof. There's a TZ suspension guru in England named Bill Howarth who had a monoshock for a TZ500 racer and said they were the same as the 750 type but shorter. So, Dwyer made some alloy pieces to adapt that shock to the Spondon frame.

Dr. Dwyer's "home operating theatre" (his small basement) includes a 13-inch Colchester lathe, Bridgeport mill, Miller TIG Welder and other professional machine shop implements. He's also aided by a copy of the hugely rare TZ service manual and microfiches. However, Dwyer reached across the pond for troubleshooting help in several areas. "There's a TZ suspension guru in England named Bill Howarth who had a monoshock for a TZ500 racer and said they were the same as the 750 type but shorter." So Dwyer made some alloy pieces to adapt that shock to the Spondon frame. Howarth also provided Dwyer with a fuel tank from a small batch he'd hand crafted. OE Gas tanks were available for twin shock TZ750's but not ones for monoshock versions like this one. "It was truly a blessing," Sean quips.

In an early go-round, he tooled down the local "straight" in first gear. "I was passing everything!"Another difficulty was finding a perch. An aftermarket producer in Florida was located who had seat moulds. "He was using the last one he made as his shop mailbox!" Since the biggest TZs are dead-wakingly noisy, "this other guy in England, who's the TZ pipe guy for two-stroke vintage racers, made this system." Well, the small cone sections anyway. "So I took 110 hours and cut and shaped the entire exhaust system," using his friend Bernstein's stock TZ750 for an eyeball template. "He and I cut and sectioned the cones, then I welded 'em up and put it together by hand." Dwyer admits to "a judicious hammer" to get some tough sections to clear. Imagine a system with 100 separate pieces (including hangers and braces) and multiple bends over and around the frame, with countless obstacles to clear. Small wonder no one makes complete TZ750 systems. Other headaches included the clapped-out, unfixable radiator. Like other factory race bits, it was nigh-on impossible to replace. Sean found another specialist to make a one-off for him. A further bugbear: the Gixxer rear wheel's sprocket proved two teeth bigger than the biggest TZ factory sprocket.

As for the countershaft sprocket, "I had one custom made, since I needed some more offset to get the rear wheel to clear. I went one tooth smaller than the factory stuff - so I really shortened the final drive ratios a bunch." He then "cleaned up everything else on the motor, made a wiring harness and battery box, and bolted a Harley Davidson accessory headlamp to the front fairing mount "'cause it's a "naked" bike." An aftermarket Triumph horn and tail lamp came from Steve Silverman at Myers Cycle Engineering. A small gel battery provides juice for the constant-loss electrics. Meantime, Dwyer had a local shop execute the trick paint job, a takeoff on the Yamaha factory scheme. For rubber pipes and odd parts, Dwyer rummaged through auto parts stores, afterwards making up "all the little bits and fittings for the brakes; and put it all together, plus miscellaneous sprockets, as I've been sorting the gearing."      

Though not privileged to ride the beast we can at least imagine the experience. Understand, first, that this machine makes 120+ brake horsepower and weighs about the same as some road going 125s. In an early go-round, he tooled down the local "straight" in first gear. "I was passing everything!" Was it loud? "You really have to keep it 'clean' and keep the revs up around six grand; so yeah it gathers some attention...." In the interest of keeping his license, Dwyer has acquired a larger rear sprocket and was "fiddling around with Boyesen petal reed-valves, two-stage jobs which I thought might make it more tractable and less of a 'light switch' motor - rather than just, you know, zero to 120 hp." Maybe a heavier flywheel would tame the TZ's manic nature? "That sort of defeats the purpose of a two-stroke," Dwyer deadpans. "I raced them for years; the beauty of a two-stroke is that once they get on the pipe, it's like a crack addict that's been let loose with a pocketful of money. It's amazingly fun."

Though not privileged to ride the beast we can at least imagine the experience. Understand, first, that this machine makes 120+ brake horsepower (Yamaha offered motors that could make 140+) and weighs about the same as some road going 125s..... At least the rims and tires are bigger than the Norton Commando-sized originals! Since TZs were banned by the FIM (Manx-like) after several years of wins, "the bikes have no real place any more in racing," sighs Dwyer. "And so although it's impractical for the street I thought making sort of a hooligan street bike out of it was a worthy mission." This leaves the ex-racer a member of a most exclusive fraternity. There are, he believes, five or 10 TZ750 streeters in the world. But only one with a Spondon frame. Unique? Close enough for us.

"And so although it's impractical for the street I thought making sort of a hooligan street bike out of it was a worthy mission."

Sean Dwyer TZ750 special

SPECIFICATIONS: Frame: Spondon road race for TZ750 Carbs: stock 35 mm Mikuni with factory jetting & reed-valve induction Crank: two-piece Front end: Suzuki 1990 GSXR750 (stock forks and brakes) Rear wheel: 1985 GSXR750, modified to accept special TZ 750 bearing Monoshock: factory TZ500, provided by William Howarth Tires: Michelin Ignition: CDI Gearing: modified countershaft sprocket, moved outboard to line up with specially "sunk" rear drive sprocket. Exhaust: hand cut, bent and welded 4-into-4, based on second factory model with expansion chambers (cones only via Swarbricks') Pistons: stock Barrels: stock Crank: stock Special plug gaps etc.

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