Cycle News is a weekly magazine that covers all aspects of motorcycling including Supercross, Motocross and MotoGP as well as new motorcycles
Issue link: https://magazine.cyclenews.com/i/125802
Lighter Than Air 10 " if l1. N ,... '" and lj o • ;g w Z W ...J () Harder to Get Hold of > () They come in yellowlblack and graylblack. both of them. Bump las opposed to kick) starting the Yam 500. by Dave Schoonmaker From the world of the bigger, faster and lighter, we bring you, courtesy of Yamaha International and Jones Motocross Products, the Yamaha 498cc motocrosser. The Yamaha 500 has been one of the least rumored bikes to come along in quite awhile. We learned about its existence only two weeks before the unveiling last Sunday at Copetown, and Gary Jones and Marty Tripes have been testing a few of these bikes at Osteen's Park for almost a month, but it seems that we were one of the first publications to develop a leak in the . cover. The local gatekeeper at Osteen's commented about the 500. "What else is old hat?" but it was exciting for us. We did get a good look at the bike and it goes something like this: The engine is a 498cc, reed valve, single cylinder, two stroke, with a four speed gearbox, set into a bascialJy stand"1"d YZ frame, with the usual special Jones running gear (foam in the rims, primary cover, etc.). The bore of the engine is a mindboggling96 mm, utilizing the same stroke as the·360 (70 mm), to squeak in under 500cc. With that incredible bore-stroke ratio the port area, from one exhaust, one intake and four transfers, is incredible. That airy cylinder is still in the experimen tal stage, sometimes being aluminum and chrome and other times aluminum with a very thin magnesium liner. When looking into the cylinder one quickly notices a strange little hole in the cylinder wall, about an inch and half below the top. I t is a compression release which is operated at all times, to facilitate rotation of the crankshfat for both starting and operation. Despite that little hole, I spent sOl)1e sadistic moments watchingDewayne and Marty break down the sales of their MX boots. One soon realizes that if these bikes were to ever go in to production, the titanium and magnesium supplier of the world would quickly become depleted. The magnesium parts start off with, of course. the hubs and extend to crank cases, the cylinder head, .the triple clamps, the rims, etc. The bike is studded with beautiful dished and hollowed titanium nuts and bolts. Other titanium parts include the handlebars, the axles, the shift shaft, and even the kickstarter lever. The stretch to the 209 pound FIM limit for Trans-AMA and Inter-AMA motocross. The engine incorporates a strange little magic box (balancer), behind and ~ Replete with oil reservoir/cooler on shocks" titanium brake arms, ad infinitum• ,. • .. " - ~ .. .! . . " above the crankshaft, which is driVe!) from the crank. The balancer, which will be found on many of Yamaha's Enduros soon, does make the bike very smooth, for what the bike is. Marty Tripes commented that vibration was hardly noticeable. Don J ones and Porter Friend (mechanic) feel that the balancer may have a lot to do with an inexactness of throule response, which is the reason given by the Europeans for rejecting the bike. They feel that they now have the solu tion and the resulting power on tap is incredible. "The bike comes on like a JAP, with the power of a 750 nat tracker, on a full load." I t was obvious from watching DeWayne and Marty ride the bikes that traction and tires will be two of the biggest problems. The engine has a vast power band and shifting is minimal. An indiscreet throttle hand produces a shower of dirt through four gears Lhat will last for indefinite distances. The power characteristics are fortunate, because a number of shifting problems have occurred, due to the varying expansion rates of alloy cases and titanium shift shafts. All three of the Jones MX trio are going very fast on the bike already and, with a few handling improvemen ts, Don feels that it will be the bike to beat. The bike has a very long wheelbase and quite a lot of rake, resulting in a desire to go straight. Problems arise in Western motocross with berms. The problem isn't a great one and Don J ones feels that "drawing it in" will solve the problem. Mike Patrick, of desert fame, was presen t and his eyes seemed to glow like neon cacti after riding it. Reliability problems are minimal after an early torrent' of seizures, caused by clearances more suited to Japanese climate and riding sty.'es. Setup as the bike is now, it runs fairly cool and is tolerant of varying jetting and much carburetor experimen tation (38 and 40 mm). The support that the Jon,:" are getting from Yamaha is impressJve, to say the least. Spare parts abound, in fact to an almost absurd extend (10 extra titanium kickstarter levers, 30 extra frames). This is a stark change from a time when Don and Porter had to manufacture their own spares. There is no doubt that Yamaha is serious and it seems that they have found the righ t people to ride their bikes in to the Oriental international motocross melee.