Cycle News - Archive Issues - 1970's

table. We knew that the Yamaha would be a little down on power - its quarter mile terminal speed was about !l mph less than the Suzuki and Honda 750·s. We hoped that the narrow three cylinder engine would give us an ad va ntage ill ground clearance. IAbove) The "stock as a stove" • well, almost - Yamaha-3.IBelow) Rider/writer Mike Bufkin does one of his things. By Mike Bufkin Photos by Jeff Harrison and Gary Van Voorhis When Yamaha first unveiled their three cylinder, four-stroke XS750 in late 1976, ' it was clearly modeled after the BMW. With a shaft drive, refined suspension, and an engine tuned for torque and gas mileage. it was a motorcycle aimed at the sophisticated touring rider. When Honda and 10 . . Suzuki looked ~t the 750 market. however, they decided that what people re~lly .wanted were 12 second quarter mile times. The result of these two approacheswas that in 1977 Yamaha had a very nice motorcycle that was about a second slower at the drags than their competition. For 1978. the XS750 was considerably revamped to match t~e C?S750 a~d CB750~ quarter . m~le times (Iosmg none of Its BMW mcenes in the process), but in the public's eye, its character was set - touring bike, not sporting bike . While Yamaha didn't seem to mind this image, some people associated with the brand did. and were beginning to resent the necessity of defending the XS750 's sporting qualities to the uninformed . So when someone came along willing to race an XS750E in WERA production road races. there was considerable sympathy for the effort, enough. in fact . to translate into real support. In March. 1978, Hasty Hom , Yamaha Motor Corporation's Southeastern Regional Service Manager, and I sat down to discuss the possibility of road racing the XS750E. I had become bored with racing a TZ250 in AMA and WERA club races and wanted a new challenge. Hasty was intrigued with the mechanical problems of racing and liked the idea of recti fying the XS750 's image. So a deal was struck in which Hasty would arrange for . the joint purchase of a new XS750E. He would tune. I would ride, and Cycle Nuts and Bolts of Atlanta would provide partial sponsorship for needed aftermarket item s. The goal was to produce an XS750 capable of winning WERA production road races . without cheating. The rules for the production classes of WERA are intended to allow a rider to bring his stock motorcycle to the track and race competitively against any other motorcycle of similar dis placement. Very few modifications are allowed and virtually none to the engine. However. once a rule is written, it is then subject to interpretation, and Hasty and I bacame very irnagina tive interpreters. The XS750 which took six firsts and four seconds in 750 and Open Production was a lot different , a lot better handling. and a good bit faster than a stock bike. But it was absolutely legal. A road racing motorcycle must have two fundamental qualities . horsepower and ground clearance. The first enables you to go fast on the straights and the second allows you to go fast in the turns. Further refinements may reduce lap times slightly, but they mainly mm.gofng fast more com£or, . The onl y place to find out was the race track. so off we went to Nashville. Tennessee. on April first for the WERA opener. The bike was as delivered except for the removal of the sidestand, center stand and tum signals. relocated foot pegs. ·fla tt er bars. and the replacement of shocks and tires with a pair of 1!l~ " S&W with 150 lb. springs. and a set of Michelin PZ-2 tires . The results were encouraging but not overwhelming. The bike won Open Production against moderate opposition, but seemed a little down on power. Ground clearance was a problem. with the soft front springs causing the bike to drag the exhaust pipes. mufflers. and the sidestand bracket. Back to the shop. Now the real work began. To improve ground clearance. the stock front springs were replaced with XSII touring pa ckage springs for a 15% increase in spring rate, and the fork tubes were extended 14" in their clamps. In addition . the ' sidestand bracket was cut off. and the exhaust pipes pulled in agains the frame tubes with hose clamps. Improvements in power were .harder to come by. The stock bike had produced a little over 45 hp on our Patraco dynamometer. Since mass produced engines typically have less than perfect valve sealing. Hasty felt that. the first step in tuning should be a good valve job. This plus some experimenting with ignition timing and spark plugs and the removal of the crankcase breather from the airbox (legal since it represented a potential oil leak) resulted in a !l hp gain at the power peak of 7500 rpm , of a 7% improvement. More important, the bilr.e was 14% above stock at 8500 rpm , which meant a substantial increase in fifth gear acceleration. Surprisingly. maximum . power was obtained with the stock ignition timing. set at 8000 rpm. and the stock air cleaner. With the engine apart. we also installed the higher capacity oil pump from the XS750 Special and the tighter Mod !l primary chain. Feeling that the bike was now pretty much dialed in. we went down to Savannah. Georgia. oil April 29 and quickly discovered that it wasn't. Despite a first and two seconds, the bike exhibited a terrible rear end pogo under acceleration, and by the end of the weekend had lost power. Ground clearance was great. but going fast was definitely not comfortable. Savannah's long straight had also revealed a problem that was to plague us for the entire season. The bike's low overall gearing limited top speed and resulted in the engine turning 9000 rpm - 1500 past its power peak. Back at the shop, the dyno confirmed the power loss. Max power had dropped back to 46 and the 8500 figure had dropped by a full 10 hp . A compression check indicated that the problem was ring sealing. So for the second time in a month. the top end came off. There was no apparant ring damage. but a check of the cylinder bores revealed that the cylinders had warped . Evidently. the extreme heating and cooling cycles produced by racing had caused the nonsymmetrically finned cylinders to reshape themselves, much as cast iron V8 engine blocks will do . The cylinders were carefully rebored and the engine reassembled, A dyno run . indicated that we had recovered the. lost power' .and. moee. ,

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