The last time we heard from Sean Skinner, he’d just built a Yamaha XS650-powered replica of the 1960s Schwinn Stingray Orange Krate bicycle. The oddball creation was a far cry from the tidy restomods that his shop, MotoRelic, is best known for—but it was no less awesome.
Once that bike was sold, Sean had an itch to unleash his inner child again, so he set out to build something even wackier. Best described as an “overgrown mini-bike,” Sean’s new whip features a bespoke frame and a two-stroke Yamaha snowmobile engine. Yes, you read that right.
Inspiration for the build came from Sean’s youth. “As a kid, mini-bikes were always in the stable,” he tells us. “Either a Honda Mini Trail or some 5 hp, friction brake death trap.”
When this nostalgia collided with a sudden interest in vintage air-cooled snowmobiles, Sean figured that using a traditional motorcycle engine would be too easy. So he scoured his local classifieds for a 1982 Yamaha SRV 540 unit—a compact two-stroke, pull-start snowmobile engine, with almost 60 hp on offer. He soon found a complete donor sled, yanked the motor out, and sold the rest of it to recoup some of his budget.
The engine was rebuilt with new bearings, pistons, and rings, and a fresh bore. In snowmobile form, the SRV 540 uses a CVT system to transfer power to the track—but this build called for a traditional manual transmission. So Sean had to devise a workaround.
“On the crank, there’s a nice tapered shaft to attach a Belt Drives, Ltd. belt drive pulley to,” he tells us. “I had my friend at RTD FAB make an adapter to mate the two together perfectly. Then, for transferring power to the rear wheel, I used a Harley five-speed transmission that was donated to the project (thanks, Dave).”
Sean disassembled the transmission, rebuilt it, and cut off anything that wasn’t needed, including parts relating to the Harley’s starter. All of the engine and transmission covers were then either vapor honed or coated in a textured black finish, leaving the drivetrain looking as good as new.
Once everything was back together, Sean fabricated a fixture to hold the engine, belt drive, and transmission together, and then welded that fixture to his frame jig. That helped him keep everything aligned, while he TIG-welded the frame tubes together and added the motor and transmission mounts.
It’s clear that Sean agonized over getting the lines of the frame right, because this not-so-mini-bike is inch perfect. A ton of work went into the front end too, where he narrowed, flipped around, and mounted the forks from Honda’s quirky ATC200 three-wheeler. Jeremy at LC Fabrications lent a hand too, by fabricating a lower yoke to help the ATC200 forks play nice with the Yamaha RD350 steering neck that Sean had used.
“I thought way too long on what to do for the front end,” Sean admits. “Most mini bikes have a continuous ‘fork’ from axle to handlebars. I really wanted that look, but coming up with a riser-less setup was interesting.”
“I ended up with a design similar to how a mirror threads onto a motorcycle. The lower parts are threaded into the fork tubes through the upper triple, and tightened into position with lock nuts. The upper part then bolts on to those, creating a truly one-of-a-kind front end.”
For the wheels, a set of vintage 16” Henry Abe mags were sourced and slapped on. The front wheel is a Harley-specific part, while the rear is meant for the Honda CB750 and is installed with custom spacers. Pirelli MT60RS tires complete the look while offering a smidgen of off-road capability.
“I have to say that I really lucked out, with the engine and trans balance center lining up perfectly with the center of the rear tire,” Sean tells us. “This made the drive chain and primary belt alignment incredibly easy, and it made the bike very balanced. Take the victories where you can!”
The bike has just one brake; a Honda CB750F Super Sport setup on the rear wheel, linked to a late-model Harley Sportster master cylinder. “I like it because it has a built-in return spring and lever stop,” Sean adds. “That made it super easy to fab up everything.”
The foot controls are all handmade parts, while the foot pegs were supplied by Johnson Engineering. They’re directly mounted to tabs that Sean welded directly to his custom frame.
With the bike now a rolling chassis, Sean turned his attention to the bodywork. A custom-made aluminum gas tank sits up top, with a section of it extending underneath the seat to maximize the bike’s range. Wes at Counterbalance Cycles added a slick black leather cover to the seat, matching the stitching pattern to the tread pattern of the tires at Sean’s request.
Finishing the bike off is an exhaust system that includes handmade sections, pieces of an expansion chamber from a Polaris, and an HVC Cycles muffler. “This engine was designed by Yamaha to have a two-into-one intake and exhaust,” he explains. “I had to extend the intake to clear the trans and make the expansion chamber completely from scratch—something I have never done.”
Dubbed the ‘Green Monster,’ Sean’s oversized mini-bike wears a sparkly lime powder coating on its frame, handlebars, and rear fender, black Cerakote on the exhaust, and a hint of dark bronze on the wheel. (Right-A-Way Powder was responsible for all of the powder coating work.)
And yes, the Green Monster is as bonkers to ride as it looks. “Pull-starting a motorcycle is not something that’s normally done,” says Sean. “Luckily the 540 two-stroke is very easy to start and doesn’t require the strength of a rugby player.”
“This bike is about how you would expect—obnoxious on so many levels. From the exhaust crackle and smoke to the open primary dry clutch noises, to the hand-numbing vibrations at full RPM. It’s definitely a handful!”
MotoRelic | Instagram | Images by Jonathan Thorpe
