We really enjoyed the standard Yamaha YZ250F but it needed some more power. With a little strategic spending we turned it in to one of the hottest bikes at the track.
After sampling the new 250F MX bikes in our 2010 250F Motocross Shootout, there wasn’t a single test rider who wouldn’t readily admitted that the Yamaha needed the most help with additional power. More outright displacement would be nice and we wanted some extra character to help make it feel special. It’s exactly comments like these that spark the interest of our friend at Jay Clark Enterprises who revels in big-bore wrenching.
“The stock Yamaha was probably the slowest out of all the 250Fs,” says MotoUSA’s regular 35+ expert tester, Tod Sciacqua. “To hear they made a 284 kit and I was pumped - the bigger the better, and this is a pimped out version.”
When Jay decides to put something together, he doesn’t just make one or two changes. Our test bike came equipped with MB1 suspension, new controls and drive gear and a whole smattering of look-good components. From the minute this thing rolled out of the van there was no doubt it was something special.
Just how did this sexy little number get to this point? Hot Rods and Cylinder Works tag-teamed the engine to punch out the displacement. A complete cylinder kit from Cylinder Works with 3mm overbore provided quick, all-in-one convenience. The larger cylinder adds just over 20cc and was complemented by a +3mm stroker crankshaft. The new crankshaft lengthens the stroke to 56.6mm and adds another 15cc by itself. An 80mm Vertex piston and appropriate gaskets are part of the kit to make sure things fit perfectly and are sealed up tight.
Tom Morgan is a name long-associated with high-performance engines. The Engineering Research and Development Manager for C&L Companies explained the benefits of the components used. Each piston and cylinder is hand chosen for each kit, ensuring a perfect match. The crankshaft is balanced to help with vibration, but the shaky nature was by far the biggest complaint from our tester.
“Although the motor had a noticeable power improvement, be prepared for some feedback,” warns Sciacqua. “It vibrated like a 1982 Maico 490! Of course, compared to the stock bike, it pulled like one too.”
A Cylinder Works 3mm over-bore kit with Vertex piston joined a Hot Rods stroker crankshaft to up the displacement to 284cc. The power improvement was drastic, but so were the vibrations from the larger motor.
Managing these new components were Hot Cams “Engine Builder” series intake and exhaust cams along with new valve springs. The cams provide higher lift, allowing more breathing for the larger motor while the springs are designed to control the valves across the rpm range while maintaining appropriate seat pressures. Even with all the modifications, the bike was designed to run on pump gas. R&D’s Power Bowl 2 carb arrangement provides an adjustable leak jet and one of their Flex Jet remote fuel screws. Clark also equipped the YZ-F with a FMF Factory 4.1 exhaust and Mega-bomb header.
The power was improved dramatically with better punch out of the corners, and Sciacqua was happy to report that it didn’t sign off on top like he suspected it might. However, our first sample of the finished project was at Piru Motocross Park, and our tester came away with a troubled look on his face. Though we consider the hillside track to be a pretty good layout for 250F machines, with few straightaways and an unforgiving hard-packed soil, the 284 just didn’t impress as much as we expected.
Smaller, hardpack tracks like Piru don’t allow the upgraded YZ-F to really shine. We enjoyed the Yamaha much better on a faster layout, understandably.
“I felt like we picked the wrong conditions for this bike,” admitted the vet speedster and longtime tester after the first day. “It would have been better at a more sandy and wide open track to let her tear up some outside lines and pound some fast, open straights.”
That being said, our next stop was the long, rough circuit of Glen Helen Raceway. Known for its steep uphills, gear-gobbling start straight and Talladega first turn, the San Bernardino track was the exact opposite of our initial ride, and much more to the 284’s liking. Coincidentally, Glen Helen is one of the favorite venues for regular vet racing, so it was perfect for our old-timer’s cheat machine. With new Renthal sprockets (14/48 gearing), our 165-pound tester was glad to have the YZ in a better moto environment.
In order to handle all the increased power, Hinson Clutch Components were sourced to beef up the clutch. An inner hub, pressure plate, steel basket, springs and new fiber and steel clutch plates were all installed. Hinson’s ultra-durable outer clutch cover tucked everything away nicely and added some additional good looks.
Handling is one of the Yamaha’s best traits and our vet tester appreciated the extra bottoming resistance of the MB1-modified suspension.
One of the things we like best about the ’10 Yamaha is that it handles so well. Amazingly quick yet unshakably stable, the chassis and suspension are easily at the top of our list. MB1 Suspension went to work on the Kayaba components with a custom re-valve. Tod, for one, is pretty satisfied with the stock arrangement. If anything tends to be an issue, as an expert-level rider, not to mention his nickname “Big Air,” he sometimes finds the limits of the stroke. Despite his belief in the capabilities of stock suspension, he was happy to have the updated version at both testing locations. Piru is a tighter, jumpier track where he was able to overshoot landings and toss it into corners. Glen Helen is all about the acceleration chop up the hills and big braking bumps on the way back down. The 284 didn’t lose any of its original stability and was actually improved because it was better suited to his faster pace. Dunlop MX51 tires front and rear made sure our YZ-F stuck to the ground.
Quick on the track and looking very quick standing still with some slick Doug Henry inspired graphics.
“MB1 does a great job with their setup,” he notes. “The bike has a lot of bottoming resistance. I would over-jump stuff just to push the suspension as hard as I could and never did feel like I bottomed it out.”
Once you get past the vibration through the Renthal Twinwall Handlebars (RC bend) and dual compound grips, riding the bike is a blast. The extra muscle is much appreciated when lining up for battle against open-class machines. However, one of the most important competitions, especially with vet riders it seems, is who has the trickest bike in the pits. Lucky for us, just getting the Jay Clark project dirty was almost worth punishment. The customized Doug Henry replica graphics from Flu Designs on white plastic bring an instant smile to anyone who remembers one of the most inspirational riders in history.
Clark went to town with the Works Connection catalog and bolted on an elite clutch perch, billet front and rear brake reservoir covers, matching engine plugs and axle blocks. A skid plate protects the big investment and radiator braces keep things straight in case it falls off the Factory II stand. A
If there’s one thing we notice about vet riders it’s that they love to bling things out. The YZ284F has that angle covered as well.
set of colored CV4 radiator hoses add some additional accents to the red/white theme. All told, the bike competes with any in terms of sex appeal. Hopefully you’re actually racing this thing in an age class, because if the big holeshots don’t draw attention then the flashy package certainly will.
“Any one of the changes to the bike would be fine, but all of them together make an awesome package. If you’ve got to have the sickest looking bike with all the whistles, this is for you,” says Sciacqua. “The biggest thing that stands out to me is obviously the motor. It puts out a lot more power than the stock Yamaha and that’s what a vet racer like me really needs because so many guys are riding 450s.”
For more information on this YZ-F project or other big-bore options in general, contact Jay at his website www.jayclarkent.com/