A 421cc two-stroke street tracker inspired by Kenny Roberts…
In 1978, Kenny Roberts became the first American rider to win a Grand Prix World Championship, beating out British legend Barry Sheene to claim the title in the 500cc premier class. Even more amazing is the fact it was Roberts’s very first season racing on the European circuits — he had to learn each track along the way. His seemingly otherworldly riding abilities prompted Spanish fans to dub him “El Marciano” — The Martian.
How did this young Californian rider manage to beat European racing legends who had whole careers of experience on fabled circuits like the Nürburgring, Silverstone, and Imatra? It was a violent riding style born and bred on American flat tracks, and it would revolutionize road racing.
“To Roberts, a dirt track racer, traction was a privilege, not a right – he was used to sliding around corners, and applied the same technique on tarmac as he did on dirt. Combined with his new cornering technique that had him leaning so deep into turns he was sliding his knee and burning holes into his leathers, the Europeans couldn’t believe their eyes. Roberts looked like a daredevil madman hanging off his bike, scraping the ground with the rear end bucking and sliding, while the experienced Europeans glided gracefully around turns. Roberts stole the show with his wild riding style, always looking like he was about to crash – but he didn’t. He won races.” —Bike Bandit
The most infamous of Roberts’s dirt track machines was the TZ750, created for the 1975 Indy Mile Grand National — a time when Yamaha was desperate to keep up with the Harley-Davidson XR750 trackers. Basically, it was a fearsome 750cc two-stroke road racing engine wedged into a dirt track frame.
On a machine considered nearly unrideable, Roberts managed to come from behind on the final lap to beat Harley factory riders Corky Keener and Jay Springsteen in one of the most legendary races in American flat track history. After the race, Roberts was famously quoted as saying:
“They don’t pay me enough to ride that thing.”
Enter our new friend Joe Banks (@wildebeest90210), who’s been building bikes since the age of 14. Today, he builds a bike a year out of his small garage. Joe has long been enamored by the two-stroke trackers of the Kenny Roberts era, but he wanted a bike he could actually ride on the street:
“I love the video of Kenny at the Indy Mile on the TZ750 flat tracker, always loved that bike. I’d seen people do the full replica with a 750/4 motor, then not be able to ride it on the road due to the nasty power delivery.”
The answer came in the form of a 350cc Yamaha Banshee engine, a liquid-cooled straight-twin sibling of the engines in the Yamaha RZ350 street bikes. He worked with renowned Banshee maestro Roger Arreola from California’s Wicked ATV for the parts to build a monstrous 421cc engine that would play nice on the street (full specs below).
As for the chassis, it’s a 4130 chromoly steel frame and nicked-plated swingarm from Geoff Cain of the UK’s Co-Built Fabrication and Design, with road-oriented geometry. That lightweight chromo frame, along with the magnesium wheels, titanium axles and assorted fixings — including one of Joe’s pride and joys, a waisted titanium rear brake torque arm — helps this two-stroke street weapon attain a dry weight of less than 25o pounds!
“Rides superbly, steering damper not required. Total weight dry 112kg.”
Of course, those 421 cubic centimeters of smoke, power, and sheer badassery come with a cost. Says Joe:
“It does have Greta on the tank delivering a stern ‘How dare you?’ It does 11 mpg on a good day.”
This is one wicked two-stroke street tracker that King Kenny himself would surely like to swing a leg over. Though Joe’s in no big rush to sell, he have the bike for sale at Sideburn if you have to have it. Below, we talk to Joe himself for the full details on the build.
Yamaha 421 Street Tracker: Builder Interview
• Please tell us a bit about yourself, your history with motorcycles, and your workshop.
I started building bikes at 14, so just getting the hang of it at 50. I’ve worked in the nuclear industry for many years now but always technical since a summer job also at 14 at a sheet metal shop. I build a bike a year from a small garage, luckily I have access to a machine shop belonging to some friends.
• What’s the make, model, and year of the bike?
Kenny Roberts replica 421cc Banshee.
• Why was this bike built?
Personal, if you can’t buy one build one.
• What was the design concept and what influenced the build?
I love the video of Kenny at the Indy Mile on the TZ750 flat tracker, always loved that bike. I’d seen people do the full replica with a 750/4 motor, then not be able to ride it on the road due to the nasty power delivery. I started with an online friend stateside gifting me a stripped motor for shipping only, then working with Roger at Wicked ATV (California) to get the right tuning parts. He understood exactly what I needed and didn’t try to sell me a dune tune on the cylinders — we’re pretty short of dunes in the UK. I worked with Geoff Cain at Co-Built for the superlight frame — from there it came together just as I’d imagined.
• What custom work was done to the bike?
421cc Serval Cub CPI barrels, stock porting, cleaned up castings, knife-edged and matched to cases
Wicked ATV billet coolheads with 21cc domes
4mm stroked Hotrods crank welded
All new gearbox bearings, modified lock up clutch on stock springs-light pull
YZ125 short kick start mod
Chariot alloy water pump impellor, straight-cut gear
Hi-capacity radiator, UK made.
50 degree c thermostat
Wicked ATV billet alloy gen cover
Power dynamo light flywheel/generator
Ignitech programmable ignition
Banshee twin coils
V Force 4 carbon reeds
Wicked ATV CNC alloy manifold with crossover
35mm PWK carbs K&N filters
Alonze engineering stainless pipes tuned for mid range with stubby cans also available
All Titanium bolts used with the exception of high tensile steel where required
Chassis and running gear:
Co-Built 4130 chromoly steel frame and swinging arm, nickel-plated arm. Road orientated dimensions, rides superbly, steering damper not required. Total weight dry 112kg
KTM steering stem/bearings
5 axis machined triples in 7068 ally
43mm R6 forks built with titanium nitride tubes, Hagon progressive springs, fully adjustable
De-lugged legs, 356 cast with new 6082 lugs welded with preheat and 4000 filler
Tarozzi fork brace
Bespoke Maxton twin shock absorbers set for bike
Marvic magnesium wheels, 70’s Morris design for TZ750
Titanium axles and swing arm pivot
Beringer classic series calipers front and rear
Brembo miniature rear master
Renthal AMA bars
ISR custom made discs front and rear
Beringer classic series front master cylinder and clutch
Domino twin pull throttle
Scitsu tacho in ‘works’ Yamaha mount
Knight bodywork in heavy duty glass Yamaha yellow gel
Custom laser cut graphics
All titanium fixings, nuts and bolts
Scar Racing titanium foot pegs
• Does the bike have a nickname?
It does have Greta on the tank delivering a stern “How dare you?” It does 11 mpg on a good day.
• Was there anything done during this build that you are particularly proud of?
I’m really pleased that it came out as it did, I’m particularly fond of the waisted titanium rear brake torque arm, I carried it round in my pocket for quite some time!
Follow the Builder: @wildebeest90210
Builder Thanks: @cobuilt_geoff @wicked_motorsports_atv