The 2010 Yamaha YZ284F Project Bike!!!!

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Motorcycle USA: Yamaha YZ284 Project Bike

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

Check out our project bike in action as it tears around the track in our 2010 Yamaha YZ284F Project Bike Video

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.”

Motorcycle USA: 2010 Yamaha YZ284

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.

Article: Motorcylce USA’s Editor JC Hilderbrand

Read More At: Motorcycle USA.com

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