Cycle News - Archive Issues - 1980's

Gary Scott's XR7S0 Gary Scott's been racing Harley-Davidson XR750s for a long time; he won the Grand National Championship for the Harley team in 1975. Since then, he's been a privateer. It's no surprise then that Scott, 32, set out to build a better XR750, paying special attention to details making the machine ~ a•• Performance in a package By Kit Palmer Yamaha's engineers were aiming at a_wider powerband when they built the 1984 YZI25L, reacting to criticism that the 1983 YZI25K's power was on-and-off, like a light switch. easier to adjust and maintain. He ended up with a new frame of his design, built to his specifications by C&]. The frame itself uses round, chrome-moly steel tubing, with a rectangular box-section brace between the steering head and the main backbone tube. The steering head runs in tapered roller bearings; the Telesco forks have 42mm stanchion tubes, and originally were for a 500cc two-stroke twin street bike made by Ossa and sold in the U.S. under the Yankee label. ScOtt added air caps to the forks. The engine cradle tubes are farther apart than the tubes of a standard XR750 frame, allowing the engine sump to be removed with the engine in place. The box-section swingarm is chrome-moly steel, running on needle roller bearings; there's a single II-inch Works Performance rear shock absorber, with adjustable rebound and compression damping; ride height is easily adjusted by unbolting and turning the bollom mount clevis; preload is adjusted by turning collars threaded onto the upper shock body. Wheelbase is 55.75 inches, rake 25.5°. Scott uses Barnes wheels and brakes and standard Harley-Davidson seat, gas tank and oil tank. The frame itself, including swing· ar' Yamaha's YZ12SL Hop-up kit: But while the l25L's powerband was widened, the horsepower needed to win was left out. For Intermediate and Pro rear axle, weighs 29 pounds. The stock H-D frame weighs 35 pounds. SCOll runs his racing program with sponsorship help from KK Motorcycle Supply, which in tum organized sup· port from ACP, Dunlop, B&S Plastics, Sunstar, MAC, NGK, Bell, Carfel, Inc., Interpart, Wiseco, KaI·Gard, Ancra and Duckworth. Scoll is also sponsored by Jack Sizemore Traveland and Deckard Equipment Co., both of Amarillo, • -.. •• riders to win on a Yamaha, the bike needs more power. The first thing people think of when their bike needs more power is a pon job; it's common for riders to take their bike's cylinder to a guy down the street and drop hundreds of dollars into poning modifications that unfortunately make their motorcycle slower around the track, not faster.. Knowing that, Yamaha Motor Corp. U.S.A. developed a YZI25L performance kit that is now on sale for $366 after extensive testing. The catch is that for enough competitive horse. power, the kit pans must be modified. The kit includes a cylinder, pipe, intake manifold, two piston and ring sets, head and base gaskets, studs and 1i_~,r.A'lV'I~." ,,_ aiAst.!.•.••.•~ .•' . .._. The kit cylinder has a chrome bore, unlike the stock cylinder's iron bore, alJd uses the stock power valve assembly. Because the bore is chromed, the kit piston and cast· iron ring must be used - the stock piston and ring will not work. Poning is completely different in design and execu tion when com pared to the stock L-model. The intake chamber - which contains the reed block - on the Lmodel cy linder is offset, the carburetor feeding the cy Iinder from an angle. The kit cylinder's intake chamber comes straight back from the centerline of the cyJinder. To use the kit cylinder, a new intake manifold with a slight bend must be fiued between cylinder and carburetor; the stock reed valve spacer must also be discarded. The standard L-model cylinder head and reed block are still used. To work with the kit pipe and cylinder, ignition timing must be retarded to 1.2mm BTDc;,,\!fOm

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