CARSON BROWN’S PROJECT YZ125 POCKET TWO-STROKE

Brown Brothers Racing (BBR) has been a pioneer in the bikebuilding world for decades now and might have just outdone themselves with this YZ125 Supermini! Yes, you read that correct, and no, they didn’t just slap a bigbore engine in a YZ85 chassis or cut down a YZ125 frame. This machine is as custom as they come!

Carson Brown at the controls of his BBR-built YZ125 Supermini.

BBR’s Duane Brown commented, “The first step was measuring and then double-checking those measurements. With complete custom builds like this YZ125 Supermini, there is a lot of trial-and-error-type modifications, but if your initial measurements aren’t spot-on, then the build is destined for failure. We are building this bike out of necessity, because my son, Carson Brown, loves ripping around on minibikes, but nothing will stay under him for very long without getting completely destroyed. So, the BBR Motorsports family teamed up to design and build something that was indestructible!”

In true BBR Motorsports style, this machine is one of a kind! The longer you look at it, the more trick details you will find.

The main frame, swingarm and entire linkage system were handmade and 100 percent fabricated in-house at BBR. The goal was to make everything look as production as possible. BBR even machined the frame cutouts to look exactly like they do on the stock YZ125 and YZ250 two-strokes. There are no OEM Yamaha YZ125 parts used on the chassis. Suspension up front is off a KTM 85, because that is the largest-diameter fork available in the 85cc/Supermini size, and then stiffer springs and valving were installed to hold up to the abuse that it would be subjected to with Carson riding it.

The rear suspension was not as straightforward as the front. A Honda CRF150R rear shock was utilized, again with a stiffer-than-OEM spring and valving setup, but BBR designed a completely one-off custom linkage system to complement the custom frame and swingarm that was hand-built by Duane. A stock YZ85 subframe and airbox were used with some minor modifications, which allowed BBR to use all the stock plastics from the YZ85, keeping the bike looking size appropriate. BBR wanted to retain the use of a stock YZ125-sized air-intake boot to ensure enough clean-air delivery to the now highly modified engine.

On the topic of performance, the Yamaha YZ125cc engine was completely rebuilt, and an Eric Gorr 134cc kit was installed that requires bottom-end case modifications. With internal engine modifications naturally comes bolt-on performance products on both the intake and exhaust sides. Intake upgrades included a Billetron 38mm carburetor from Lectron featuring metering rod technology and a complete Moto Tassinari VForce 4 reed block to supply the Maxima premix-infused race gas. On the exhaust side, an off-the-shelf Pro Circuit complete system was used with a Works-style pipe and R304 Shorty silencer for a couple of reasons. Performance, of course, but also because it would be easier to replace than a custom cone pipe when Carson demolished it—and that isn’t a matter of if but a matter of when.

A custom aluminum gas tank, an aluminum frame built from billet with dimensions based off a big-wheel YZ85, hand-built swingarm and BBR linkage are just some of the things featured on this insane build.

We mentioned before that the gas tank was custom-made, so besides the obvious cool factor, the major reason this was done was to allow the use of a two-radiator system to cool the hopped-up Eric Gorr engine racer versus the one-radiator system that the stock YZ85 tank was designed to work with.

Other notable bolt-on items on this BBR YZ125 Supermini included a complete wheelset built by Kristen Anderson and the crew at Dubya USA, featuring billet-aluminum hubs, oversized spokes and Excel Takasago blue rims wrapped in Dunlop MX34 tires. MotoStuff upgraded the braking system with an oversized rotor, billet-aluminum brake caliper up front, and a heat-treated stainless steel rotor in the back. MotoSport Inc. was BBR Motorsports’ main source for all OEM parts needed on this build.

The Dirt Bike staff faces’ light up when looking at their caller ID and it displays Carson or Duane Brown’s name, because it almost always means we are going to be involved in something fun and probably never something we’ve done before. With over 300 man-hours put into just the labor on this YZ125 Supermini build, you can probably imagine that there is no way BBR Motorsports can offer this type of build to the general public, but it’s amazing that they were able to pull it off in the first place. If you want to see Carson Brown on this and more custom builds, head over to Dirt Bike Magazine’s YouTube channel now!

The YZ125 engine fits so well inside the custom BBR Motorsports frame that at first glance we didn’t even realize it was a 125cc engine.

CARSON’S TAKE
This is a complete one-off Supermini-sized bike that we put a YZ125 engine into. I have tons of fun ripping Supermini-sized bikes, but struggle to keep them alive as an adult. We talked about just building an aluminum-framed Supermini to keep the frame from bending and breaking everything around it. But, we had also talked about putting a 125 into a Supermini so that it has tons of fast, fun and reliable power. My dad (@duanebrown_bbr) got to work on this dream project by putting in over 300 man-hours of milling, welding, machining and grinding to make this thing a reality.

An off-the-shelf Pro Circuit YZ125 exhaust system was used for a couple of different reasons: One, it made fabrication of the mounts easier, and two, it’s not a matter of if but matter when Carson Brown smashes the pipe it will be way easier to replace.Motorsports frame that at first glance we didn’t even realize it was a 125cc engine.

This is not a cut-down 125 frame; he made the frame from scratch. Nothing off of a YZ125 frame was used. By the end of it, he made a full one-off aluminum Supermini frame, linkage and swingarm while rocking a YZ125 engine with an Eric Gorr 134 kit. We made sure to make the YZ125 air-intake boot fit so that it didn’t lose any power, along with the YZ125 dual radiators so that it stays cool. We are running KTM 85 forks/clamps since they are the biggest diameter around out of all the 85s. It still runs the stock YZ85 subframe, seat and plastics with a custom aluminum gas tank (the stock YZ85 tank only has enough room for one radiator).

The bike has a Pro Circuit (@procircuit78) YZ125 pipe/silencer; Dubya (@dubyausa) 16- and 19-inch wheels with Dunlop (@ridedunlop) MX33s; MotoStuff (@motostuff_com) oversized front brake/rotor; and the Lectron (@lectronfuelsystems) Billetron, along with Maxima (@maximausa) oil. Huge thanks to MotoSport (@motosportinc) for helping to get all the parts needed. We made this bike purely to have tons of fun on, and this bad boy is always an absolute blast to ride!