Cycle News - Archive Issues - 1970's

compression piston, the engine is ri; iont brake is from a YZ250D; bigmotor over; Gianatsis gets the . and gas/oil Ohlins give 11 of W throttle di aled on th e fron t end could be made ext remely light and brought up to clear an y whoops or bumps. The power is really fun . On most track su rfaces I don't think it will out accelerate a two-stroke, but the torque is so strong it wants to jerk your arms out of their sockets every time you get heavy handed with the throttle . Bengt said I was trying to rev the motor too quickly . . . like a two-stroke engine .. . and that the throttle should be rolled on slowly coming out of corners or accelerating. Too much throttle all at once he said. will make the motor try to bog and not pull as smartly. The motor is basically stock Yamaha with only two major modifications. Compression has been raised to II : I with a ProTec piston. and a 36mm Mikuni is used for carburetion. The straight pipe which Hallman provides allows enough power that th e stock cam with its broad powerband is reta ined . There's probabl y not another motocross bike around that . literally. pulls from id le like the Hallman/Yamaha , nor will run an entire season without a top end job. Compression braking is another advantage of the 500 cc fou r -stroke that allows it to get away with using lightweigh t brake assemblies like those off the YZ 250D without ha ving problems getting stopped for a tu rn. There ar e times though , when the 500's extra weight does seem to be a d isa dvantage . It usually becom es apparent while slamming across a section of rou gh whoops. Rather than try and bounce across the tops of th e bumps like lighter bikes wou ld , the 500 wants to ram int o eac h on e. This is where the use of top quality suspension com po ne nts becomes im po rt a nt. a nd in th e case of Bengr 's bike th e Ceri ani forks and Ohlin shoc ks really proved th eir wort h. Also. th e bike proved to be a handful in dee ply rutted turns where the weight was difficult to jockey around to a smoother line. Is th e 500 H allman/Yamaha a competitive motocross bike? That's a difficult question to answer. After I rode the bike, Roger DeCoster took it out for a few quick laps around the Trans-AMA cou rse at St . Louis. He was equally impressed with t he performance and handling, but also had a difficult time trying to form an opinion of it . Bengt Aberg proved it could win in a World Championship Grand Prix against exotic factory two strokes, but I think much of it depends upon the track conditions. Four-stroke engines seem to be best suited for rolling hill type tracks with grass or loam surfaces. These ' are the kind of conditions found in countries such as England where fou r-strokes remain qu ite popular. In t he recent British Na tional Motocross Championships , Bob Wright used his 500 CCM fourstroke to beat out Graham Noyce rid ing a 440 Ma ico at the final race of the series , though Noyce still had enough po ints to retain his title. On horsepower tracks like southern California or rough Eastern tracks like Unadilla , the quicker and more agile two-strokes would have th e upper hand . Two-strokes even seem to have th e advantage on mud tracks where their qu ick re vving engines ca n continuously spin clea n the mud off knobby rear tires. Bengt was forced to retire from th e muddy Red Bud, Michigan. T rans-AMA race when he kept falling do wn because his rear tire kept pac king up with mud . But for its seeming disadvantages, t he 50 0cc Hallman /Y am aha is probably th e neat est bike you coul d ever own. It 's mo re fun than an y other bik e I've ever ridden , and seems th e ideal machi ne for the semi -serious racer who might want to use it for trail riding one weekend rather than motocross all the time. Certainly nothing can be more satisfying than building your own unique race bike up from nearly scratch, except perhaps feeling and hearing it roar to life the first time you d iscover that secret starting spot before top dead center (DeCoster cheated when he stalled the bike and push started it) . If I could have my choice of owning any bike in the world - from DeCoster's RN 400 Suzuki to Smith's Honda RC 400 . I think I would still choose Bengt Aberg's Hallman/Yamaha . The excitement and pleasure of riding such a bike can probably never be equalled by any other dirt machine. Originally Torsten Hallman Racing was going to offer the thumper as a complete bike, but with all the top components required , the price for a finished motorcycle could add up to as high as $3 ,500 . So what Hallman is doing now is selling the Bengt Aberg replica as a frame kit and assorted parts. Depending on what you spend for a new or used Yamaha 500 TT, the number of replacement com po nents you add to the kit. you could build your own Bengt Aberg . grand prix winning replica for as little as $1 ,500 or as much as $3 ,000. Below , you'll find a price breakdown of the more important parts that are available. For more complete inform a tion contact: Torsten Hallman Racing 1300 Hill Street EI Cajon, California 92020 (714) 442-043 I. Frame assembly $745.00, Brake pedal 29.40, Exhaust pipe 92.25, Chain tensioner 21.75, Rear axle, complete 61.75, Fox Shox 14-% in. 109.00, Fox AirShox 17- % 290.00, Simons forks 325.00, Gunnar's throttle 17.95. • 25

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