MXA’S MAMMOTH MOUNTAIN KTM 65SX SPECIAL

Mammoth Motocross has been a staple for youth, amateur and vet racers for 54 years. The special opportunity to race in the mountains, on a track that is only open for one week out of the year, has for decades drawn riders from all over the USA to test their skills against one another. With the race being held at 8000 feet above sea level, Mammoth offers unique challenges for racers and their machines alike. Now that fuel injection has become the norm in the big-bike world, it has alleviated some of the high-altitude jetting issues. But, for the carbureted 65s, 85s and Superminis, the jetting is the first hurdle to jump. MXA decided to build a carbureted KTM 65SX for our test rider to race against some of the best amateurs on the West Coast.

The biggest challenge for the machine is the elevation. Thinner air obviously impacts a carbureted engine. For solutions, we turned to Dean Dickinson from R&D Racing and asked him to set up our KTM 65 engine. Dean has years of experience building engine packages for all types of bikes and is especially skilled at tuning an engine that’s fast and durable. On MXA’s KTM 65 Special, Dean installed his custom carburetor needle and valve-spring package to lean the bike out for the altitude. This, coupled with his special ignition timing, created the perfect package for Mammoth. From the first start, we knew the bike was ready to go without even having to touch as much as the air screw.

The next obstacle for high-altitude running was the right fuel and premix. It does not matter how great the engine is. Without the perfect fuel/oil ratio, it goes back to square one. Some racers choose to use oxygenated fuel from the local gas stations, while others go a more exotic route. For our project bike, we turned to our friends at ETS and Blendzall. ETS recommended XPO 105, which is a high-octane, oxygenated fuel combined with Blendzall’s Ultra Racing Castor mixed at 44:1. 

JUMPING THE LAST HURDLE

The last hurdle when putting together the best engine package for Mammoth is getting enough air into the engine and venting it out with the best exhaust pipe. The higher the elevation, the thinner the air. And less air equals less power. To counteract this, MXA chose to run a Twin Air filter with Pro Circuit’s KTM 65SX air boot and exhaust system. Twin Air’s filter increases airflow while also sealing to the stock airbox to keep dirt and debris out. We chose Pro Circuit because of the unique air boot they make to fit between the carb and the airbox. Compared to the stock air boot, Pro Circuit’s air boot is larger and is designed to optimize the airflow into the intake. As a cherry on top, we added PC’s Works pipe and the R-304 stainless Shorty exhaust.

Once we had the elevation and engine situation figured out, we began focusing on the rest of the bike. The Mammoth track is not known for its jumps, but for its famous uphill and downhill sections. The track tends to get chattery rough as the weekend riding takes its toll on the dirt. To make sure we had the right suspension settings, we turned to the ladies and gents at Research Engineering Performance (REP) of AEO Powersports. REP is an in-house suspension and tuning service located at AEO’s Murrieta, California, shop. Mark Johnson and his team have a long history with KTM, so it was a no-brainer for them to set up our KTM 65SX’s suspension. We answered a few questions about the rider’s height, weight, and skill level to get started and boom! We didn’t even have to adjust the sag! The suspension performed great on the mountain.

GOING SMALLER FOR SMALL RIDERS

For the controls of this mini steed, we elected to go with a few partners that make quality products for smaller bikes. For handlebars and grips, we elected to go with Pro Taper. We chose their Fuzion bar with the Carmichael bend and half-waffle soft grips. Most of their bars come stock at 800mm wide for the big bikes, but these bars were easily cut down to fit our smaller rider, thanks to convenient measurement marks. To ensure we wouldn’t leave broken levers all over Mammoth Mountain, we installed ASV’s F4 clutch and brake levers. ASV has a Shorty option for smaller hands. Trust us, our test rider put these levers to the test over the weekend. Last, we mounted a T-Rex Moto-X throttle assembly and upper triple clamp. The race-spec throttle assembly comes with a more robust throttle cable, a billet throttle housing and a billet throttle tube, providing an easier twist of the throttle and added durability. The anodized chrome finish looks pretty rad, too. 

As for the upper triple clamp, compared to the stocker that comes on the 65SX, the T-Rex upper clamp has a four-bolt setup and is much sturdier and more rigid. This limits the flex when it comes to handling and diminishes the twisting of the front end in case of a crash­—another must for our little ripper.

WHERE THE RUBBER MEETS THE ROCKS

Mammoth dirt is unique in how it will change throughout the day. Early in the morning, because of the cool mountain air, the dirt holds the moisture really well and is almost sandy; however, as the sun rises, the track dries out and becomes more hard-packed. One thing that remains constant at Mammoth, however, is the rocks! The rocks create another layer of difficulty for the 40 racers on the track. To handle the elements, we went with a Maxxis MX-ST tire combo. We figured that if Jeremy McGrath approved when he swept all four pro motos, our mini test rider would do just fine. Our tires were wrapped around NacStar USA’s new hub and Excel wheel combo. As you can see, not only did the wheels handle the abuse of our test rider and the conditions, they look pretty good, too.

Did we mention rocks? We knew that for the bodywork of our 65SX Special we were going to need plastic that could survive rocky roost and still look good. Our all-orange machine popped, thanks to Acerbis plastics and Roost MX graphics. We went with the Acerbis all-orange plastic kit and MX Uniko Vented handguards, a much-needed accessory at Mammoth Mountain. With Roost MX taking care of the graphics, we opted for their Ballast FB design and asked for all the logos to be black. This provided the necessary protection we were trying to achieve, but also the overall sleek look we were going for.

Our test rider, Nixyn D’Errico, definitely identifies with the “look good, feel good, race good” mantra. So, to make him feel even racier, we turned to Nihilo Concepts for parts that are specially built for Austrian machines. We installed a reflective gas-tank heat-shield tape for the fuel tank, high-pressure radiator cap, custom-engraved ignition cover, frame grip tape, case saver and oil plug. These parts were the icing on our KTM 65 cake.

There is a definite science to building a competitive and reliable carbureted two-stroke to race at the historic Mammoth Motocross race. The high elevation, coupled with the rough and rocky terrain, presented unique challenges for us. Fortunately, we were able to partner with great companies who provided strong products and services to ensure Nixyn had a race steed that could handle the elements. Our KTM 65 far exceeded his expectations when it came to power, handling, suspension and reliability for this once-a-year race. The bike was put through its paces over the three-day event, from ripping a holeshot to navigating the steep downhill at speed and even holding up as the bike and rider hit the deck a few times. At an event that can cause bikes to be very temperamental and a mechanic’s worst nightmare, we are proud to say our KTM 65SX project bike gave us no issues.

 

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