YAMZUKI RGT 421 CHEETAH

YAMZUKI RGT 421 CHEETAH

by Bauer Xcel |

SEAN ADAMS HAS ALWAYS WONDERED WHAT A 350 VERSION OF THE YAMAHA TZR WOULD’VE LOOKED LIKE. FUSING PARTS FROM REMOTEST SCOTLAND AND AMERICA, MEET A HYBRID THAT’S 30 YEARS IN THE MAKING

YAMZUKI RGT 421 CHEETAH

You can't say Sean Adams isn’t organised: he’s been planning this Special Brew for 30 years. It was back in 1986 as a young racer in the 350 Production Class that he looked on enviously as the Yamaha TZR250 came out. “All of a sudden there was a 250 that was nearly on a par with the 350: the handling was better and the power wasn’t far behind,” recalls Sean. “A lot of us at the time were thinking, ‘When Yamaha bring out a 350 version of a TZR, it’ll clean up.’

Sadly, it never came. Over the years I’ve wondered where Yamaha would’ve ended up if they’d have carried on with 350 twostrokes, and this is my interpretation.” Making a hybrid of such ambition required an idea, and Sean immediately knew the path he wanted to go down: fitting a Yamaha RD350 YPVS two-stroke engine into a Suzuki RGV250. This craziness required two donor bikes, which turned out to be from differing ends of the quality spectrum. “The first thing I bought was a 1993 RGV250P off eBay, which was in about the most northern part of Scotland there is. Any further north and it would’ve been in the North Sea. I bought it for £1700 but I could see why no one wanted to go up there – it took three months to collect it because the owner was so remote he basically got snowed in.

I eventually got hold of it in March 2011.” While the RGV was, to quote Sean, in “belting condition”, the YPVS he bought for £400 to get hold of the engine was certainly not. “I just needed the crankcases, gearbox, clutch and wiring loom, but the bike was a state. The guy couldn’t get it running and I wasn’t surprised – he’d actually put the powervalves in back to front.” The previous owner’s clottishness wasn’t all bad, though – the engine and barrels were in great nick and quite a bit of cash had been spent on the top end. After the purchase of a CP Industries engine kit to beef things up,

LOOM

Despite all the modifications to the engine and frame, the area that caused Sean the most head-scratching was the wiring loom – and bear in mind that this is a man who spent a lot of time in his r acing days tinkering with looms. “I wanted to use all the Suzuki switchgear and lights, but with the Yamaha wiring loom,” he says. “Planning how to make those two things work together took a lot of time, because on the Powervalve everythingis under the fuel tank on a plastic tray above the cylinder head, but I wanted it all in the seat hump. There was a huge amount of electrical stuff to relocate and it took me the winter of 2013 to do it.”

YAMZUKI RGT 421 CHEETAH

NEW SPROCKET

“I got a new, one-off sprocket made but when I finally got it on, the chain hit the expansion chamber,” smiles Sean, rolling his

eyes. “It would’ve sawn a hole straight through. I had to get Barry at Blue Haze to modify it, which took another

few months. But once I’d got the clearance right it went to the dyno and made 80bhp straight away.”

WHEELS

Sean went for GSX-R400 wheels front and rear, which had two major benefits: firstly, Sean felt a four-stroke wheel would handle the extra power and torque, and secondly they were about half the price of RGV wheels. As a bonus, the front went straight in.

BODYWORK

Hastily ditching the bike’s Lucky Strike paintjob, Sean decided he wanted to design something new – so with the help of Steve

Puffer at Barwell Bodyworx, he got cracking. “I wanted the classic Yamaha red and white, but I took a bit of inspiration from one of Colin Edwards’ colour schemes from MotoGP,” admits Sean. “It’s Tyga bodywork and the only thing I had to cut down was the bottom half of the fairing – I had to make a slot in one side to take the Yamaha clutch case because it’s a bit wider than the Suzuki’s. I had to cut it back quite a bit at the bottom to clear the pipes, too.”

YAMZUKI RGT 421 CHEETAH

EASTERN PROMISE

“I got a few bits like the rear lights, seat, rearsets, top yoke and clocks from Tyga in Malaysia, rather than pay someone to make them for me,” says Sean. “Also, because they sent them to me as ‘sample motorbike parts’ I paid no VAT or import duty.”

YAMZUKI RGT 421 CHEETAH

FORKING OUT FOR QUALITY

Sean decided there was only one company to work on his forks – Maxton. “I stuck with RGV250 forks – I thought they’d be best seeing as they were built for the bike,” says Sean. “I figured that once Maxton had worked their magic on the internals, they’d be a better set of forks than I am a rider.”

YAMZUKI RGT 421 CHEETAH

WHAT’S IN A NAME?

We’ll let Sean explain the name ‘Yamzuki RGT 421 Cheetah’: “Yamzuki because it’s a Yamaha/Suzuki hybrid, RGT because it was an RGV but the V for the V-twin has been replaced with the T for the parallel twin, it’s 421cc and Cheetah is the name of the CP Industries cylinder kit on it.” Simple, eh?

YAMZUKI RGT 421 CHEETAH

ENGINE

“I decided from the start that I was going to get a tuning kit with big-bore barrels for the engine,” says Sean. “You get them from America and I knew from other people who’d tried them that it was a proven route to go down.” The CP Industries engine kit duly arrived and, while it was one of the project’s biggest expenses, it was just the job. “It came with a longstroke crank, a set of full 421cc barrels, new powervalves, reed blocks off a CR500 that go in the back of it, forged pistons, inlet rubbers, and all the nuts and bolts,” says a clearly chuffed Sean. “It had everything I needed to build the engine.”

YAMZUKI RGT 421 CHEETAH

WORTH EVERY PENNY

As part of the CP Industries engine kit, Sean got these pneumatic powervalves. “They work with exhaust pressure rather than an electric system,” he explains. “The engine parts were expensive, but worth it.”

YAMZUKI RGT 421 CHEETAH

COMPLETELY FRAMED

The intricate framework was carried out by Nigel Kimber at NK Racing, who Sean describes as Britain’s “Mr Two-Stroke Chassis Modification Guru.” The engine ended up fitting more-or-less straight in.

YAMZUKI RGT 421 CHEETAH

Sean stripped down the Powervalve motor and shipped it all to Stan Stephens to rebuild (“I went the chicken’s route because I’d heard a lot of people had problems with piston-tocylinder clearance”). You might think that getting the YPVS engine in the RGV frame would be an utter nightmare, but it turned out to be surprisingly easy. “It needed a new engine cradle for the front, which was made by Nigel Kimber at NK Racing,” says Sean. “He also reformed the rear top engine mounts, cut the RGV ones off and put some new ones on to suit the Yamaha motor, and also made a new subframe to carry it. It was a big undertaking. You’d have thought that starting with a chassis housing a 250 V-twin and then fitting a 400cc parallel twin might cause an issue, but it’s almost as if it was designed to go in.”

Next up were the beautifully welded exhausts. Well, sort of. Sean had identified Barry Dawson at Blue Haze Engineering as the man to fashion his new pipes, but regular readers of PS will know that plenty of others regularly do too – hence Barry’s six-month waiting list. “I thought it would be fine and give me time to sort the wiring loom out,” says Sean. WRONG. Taking the Powervalve loom, unwrapping it and planning how to wire the Suzuki switchgear into it redefined the word ‘nightmare’, and with one thing and another meant that Sean missed his slot with Barry. “I had to go to the back of the queue and wait another six months,” he laughs.

Sorting things like the forks and frame in the meantime, the pipes finally got done – and it was worth the wait. “He’s an absolute no-nonsense perfectionist,” says Sean. After a brief modification to the Tyga bodywork, the moment had arrived for the first fire-up. There was a big drum roll... Sean turned the ignition... and nothing. It turned out that the generator that came with the bike had failed, so with that sorted he tried again... but still no spark. “I was trying to work out what the hell was going on when I accidentally kicked it over with the kill switch in the ‘off’ position,” laughs Sean. “It sparked first time. It turns out that the Yamaha kill-switch works differently to a Suzuki one – one works on open circuit, one works on closed circuit. What I thought was some complex electrical problem was just the flick of a switch.”

Having started the project in March 2011, Sean finally finished it at the end of last year – and has spent the intervening months enjoying it. “I like doing hybrids because there are no constraints,” says Sean. “No one’s going to say, ‘The nut wasn’t that colour on the 1985 model.’ For 80bhp and 120kg, it shifts. I built it to ride, not to look at.” Next on Sean’s list is the modification of a Testi Champion Veloce Special – a rare moped with a monoshock chassis and a Minarelli engine. We know how Sean likes to take his time if the 30-year gestation period of the Yamzuki is anything to go by, so we look forward to featuring it some time in 2046.

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SPECIFICATION

YAMZUKI RGT 421 CHEETAH

YAMZUKI RGT 421 CHEETAH

ENGINE 1986 RD350F2 two-stroke parallel twin, CPI 421cc Cheetah kit with pneumatic powervalves, Hot Rods welded crankshaft, V Force reeds, 34mm Keihin PJ carbs, Blue Haze pipes with carbon silencers, uprated clutch plates with heavy-duty springs, Zeeltronic programmable ignition, Dyna ignition, lightened flywheel, Yamaha R6 radiator, Kartelli inline thermostat, uprated water pump, silicone hoses

CHASSIS Suzuki RGV250P frame and swingarm, box section aluminium engine cradle and frame modifications by NK Racing, Tyga subframe, RGV-P forks with anodised legs and Maxton internals, Tyga yokes, Maxton RT10 shock, Suzuki GSX-R400SP wheels, Michelin Power 3s, 120/60 17 front, 160/60 17 rear, Galfer front discs with Hayabusa six-pot calipers, Brembo mastercylinder, Goodridge lines, GSXR400SP rear disc, Suzuki RGV-P caliper and mastercylinder, Tyga rearsets, Tyga fairing and seat unit, Suzuki RGV-P front mudguard and tank, Harris clip-ons, Ohlins steering damper, one-off fabricated electrics tray, coil and radiator brackets, Koso speedo/tacho, modified Yamaha RD350F2 loom, Suzuki RGV-P switch gear, Tyga lights, Honda battery strap

WORDS HANS SEEBERG PHOTOGRAPHY JASON CRITCHELL

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