Thumper Racing/Hot Cams CRF300R Project

The 2010 Honda CRF250R is an amazing motocross bike that managed to top our 2010 250 Motocross Shootout with a host of standout characteristics and new technology. Adding fuel injection was the big news and the engine churns out smooth, usable power and harnessed in a lightweight chassis that makes getting around a moto track almost as fun as it can be. Almost. Motocross riders are a group that’s never satisfied – bikes aren’t fast enough, suspension isn’t smooth enough and gear isn’t cool enough. For as good as the stock CRF is, we enlisted the help of Thumper Racing and Hot Cams to build us a machine that brings even more to the table. The whole project was orchestrated by Jay Clark at Jay Clark Enterprises and he made sure the rest of the bike was tuned to match with custom graphics, MB1 suspension work and details from front to back.

As the 2010 Fire/Police Motocross Grand National Champion, Ryan Chapin is the type of rider who can make use of a project like this. The Open Expert and 18-39 Expert divisions where he claimed championships allow for open displacement motorcycles, so a cheater bike like this isn’t actually cheating at all - its smart. As an active vet racer, this is just the type of bike that can provide 250 handling with that little extra oomph to get out of corners and over jumps around the motocross track. Chapin rode a Yamaha YZ450F to his titles this season and knows the benefits of a powerful engine. While we were busy conducting our 2011 450 Motocross Shootout, we turned the Los Angeles firefighter loose on the Racetown 395 track with a stock CRF250R for comparison.

We've already seen what's possible using the stock cylinder with the Hot Cams Honda CRF256R Project Bike. That test showed the available overbore from Honda’s aluminum cylinder is good, but not necessarily enough to get major gains. In order to go bigger, Thumper Racing drops in its own billet aluminum cylinder sleeve which adds strength and uses the “Cool Flow” design with a claimed 25% more water/cylinder contact for better heat control. Thumper also modifies the case to make room for more stroke and the final dimensions allow for an 81mm piston and 58mm stroke which nets almost 299cc.

The 300 conversion is sold as a complete kit so that Thumper Racing can handle the case modifications and crank balancing themselves. TR does all the labor while CP Pistons provides the box-style piston with 12.5:1 compression, Cometic handles the gaskets and the extra stroke comes via a Carrillo rod. There’s no doubt that adding cubic centimeters is going to beef up the power, but big-bores are traditionally known for juiced bottom end and midrange. In order to keep the CRF’s powerband similar to stock with a smooth pull and plenty of over-rev, a Hot Cams 1160-BLD cam was installed which helps keep the ponies coming as the rpms climb. Our tester was immediately impressed with the throttle response and overall robust character of the modded engine.

“The Thumper Racing motor feels stronger all the way through the rpm range,” confirms Chapin. “The biggest benefit was that it kept pulling on the top end and kept generating power. Jay had to tell me to rev it out a bit and ride it less like a 450. That power helped eliminate a couple of shifts every lap compared to the stock motor. I enjoyed being able to lug it like a 450, but still keep the wheel spinning like a 250F. The motor was very free-revving with instant throttle response. The stock 250 motor felt choked in back-to-back tests. It’s no contest between the stock and mod motor and it would be fun to race with 450s for better comparisons.” 

VP Racing Fuels U4.4 gasoline brings out as much performance as possible. VP designed this fuel to work specifically with modified engines and resist the effects of heat near the fuel tank. Doling it out is a revised throttle body courtesy of Injectioneering, and burnt fuel passes through the titanium FMF Factory 4.1 RCT MegaBomb exhaust system. CV4 silicone hoses add style and fuel line heat wraps around the clutch cable to protect it from the hard-running engine. At one end the cable is connected to a Works Connection Elite clutch perch and Hinson clutch components at the other. Underneath the outer clutch cover is an inner hub and single-spring pressure plate kit. The freshly prepped Racetown track put the clutch to task, especially early in the day, but a red anodized Ultralight Renthal sprocket lays down the added power.

“The Honda transmission performs almost flawlessly,” continues Chapin. “There’s a bit of a gap between third and fourth gear that I think can be fixed with one tooth on the rear sprocket. I had trouble with the new clutch fading a bit, however the Works Connection perch was priceless in feel and in adjustability.”

No to be outdone, MB1 got ahold of the suspension and went to work on polishing things up. First is a stiffer set of 0.48kg/mm fork springs and a full revalve on the front end. Out back they revalved the shock and installed a lighter spring (5.2kg/mm) which helped give the CRF better overall balance. Our tester was quick to praise the new components with keeping the bike settled while hammering obstacles. Braking bumps disappeared and 395’s big 100-plus-foot step-up was no longer a concern for sore joints (of course it was much easier to clear with the extra horsepower).

“There always seems to be a tradeoff with stock plushness and modified bottoming resistance,” muses the FPMX champ. “The stock suspension handles small chop perfectly but is downright scary on big hits and harsh landings. The MB1 suspension inspires confidence when pushed hard and especially charging turns with big braking bumps. It’s very controlled in over-jumping and under-jumping situations. The spring changes kept the front end up in the stroke and made the bike very level. I tend to run my 450 low in the rear but this level feel was great on a 250.”

In addition to the revised suspension, Clark also bolted on a blue CRF450R linkage pull rod from Suspension Direct (part # SDECPRH09) for a more linear curve. The suspension changes make the CRF a well-mannered machine, but Chapin also likes the benefits of a wider profile on the Dunlop Geomax treads.

“I loved the Dunlop 120 tires. Flat corner traction is incredible with a much more planted feel,” he says. “With the Thumper Racing motor there is adequate power to pull these tires. I really thought it would bog the motor down too much with the bigger contact patch, but with a touch of the clutch the tire would spin again easily.

“The ergonomic combination made for great comfort,” he continues, referring to the Renthal Twinwall handlebars, bar mounts and dual-compound grips. “Being 6’1”, personally I would like to open the cockpit with a top clamp and maybe lower footpegs, but I would be afraid of grabbing my boots in ruts.”

Another benefit of the 250F machines are their lighter weight and impressive braking prowess. Honda has always been known for having good binders, though the rest of the class has definitely caught up in recent years. In order to give the red ripper a new edge on the competition, a QTM 270mm oversized brake rotor is installed on the front.

“I like progressive, one-finger control without the brake being too grabby. I like to run a stock brake line with an oversize rotor to keep the squishy but strong feel, and the QTM oversized front rotor provided me one-finger stopping power.”

Like all of his projects, Clark threw the book at this one with plenty of extra components and upgrades like TCR wheel anodizing, Works Connection engine plugs and a semi-custom set of DeCal Works graphics and preprinted numbers. It sure looks the part, and the total pricetag is just as sharp. But when the result is a bike that offers the best of the motocross world, it’s hard to complain. Honda’s CRF is already one of the best-handling machines available and MB1 tightened up the suspension even more. Our tester even said the engine should be stacked against a 450 just to be fair, and all the extras make this feel like a high-quality race bike. If giving the 250 in your garage some extra life is a top priority, especially if it's already due for a rebuild, Thumper Racing has it covered.