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/126395
Impression: Yoshimura GS1000.. un erl By Pat Eagan Photos by Patrick Behar The ·" P op s" Yoshimura -prepared Suzuki GSIOOO, in the hands ofWes Cooley and Steve ' McLaughlin; was the most competitive bike in AMA Superbike Production racing for 1978. It won or led every race it entered. Mc Laughlin d rew the last starting position on the grid at Daytona but his Yoshimura GSIOOO was the first bike to receive the checkered nag . At Pocono , after a race-long d uel , Wes Cooley beat Mike Ba ldwin's Du cati, whi ch was later disqualified for non -standard engi ne cases . It was a much tighter contest at th e FIM ra ce a t Laguna Seca, Rig ht behind W es a t the checkere d nag were Paul Ritter and McLaughlin . It was close-but -no -cigar for W es and Pops at th e sp ri ng Loudon event where th ey led until the oil cooler broke from vibration . Wes stretched out his lead a t Sears Po int until th e tremendous ' to rq u e of the Yoshimura e ng ine sheared the bolts that held the sp roc ke t on th e rear Mo rris mag wh eel. Raw , b ru te power is th e underl yin g the me of the Yosh imu ra Su zuki . Power that shea rs sprocket bolts when no one else ha s such a problem. Power th a t seems to pull the asphalt towa rd you ra ther t ha n you toward. it. If ever of lea ping tall buildings in a single bo und , thi s is it! Po p s h a s modified the motor .extensively; no a rea was overlooked . Excessive hea t is the enemy which can mortally wound a strong motor. T he first ste p is to have it sandblasted and trea ted by Kal -Gua rd for better hea t dissi patio n . To f urt her im p rove cooli ng , holes have been drilled down through the fins of th e head and ba rrel. The oil coo ler is now rubber mounted to the frame downtubes just below the st eering stem . It is manufactured by N ip pondenso a nd fed by a full -now fitting . T he fitti ng takes the place of the cover wh ich held the oil pressure warning light switch on the stock GS. Yoshimura pistons are pressure-cast (which is stronger than normal casting and less prone to heat .induced distortion and seizure than for gin gs). The com pression ratio is 11.2 to 1 a nd remains the same for all tracks. Only one com p ression ring is used . Stage II camshafts open the oversize va lves t in th e highly modified head. The int a ke valves a re one mi llimete r oversize at 39mm wh ile t he exha usts e sc of two millim et er s. To im prove gas n ow, the valve stems ha ve bee n red uced fro m seven to 6.6 m m . Val ve clea ran ces are adj usted in a sto ck GS mo to r by c ha ng ing shims o n to p of valve bu ckets. At hig h rpm, th ese sh ims ca n be eje cted .from t hei r buckets ca using d amage to the ca mshaft . valve cover . and cyli n de r h ead . Pops u ses a differ ent set -up . p lacin g the shims u nd e r th e valve b uckets, ' t he re by eliminating th e possibil ity of such a mishap. OO A weak link in high rpm GSI O s has been the' ca m chain. Pops has co m p letely re -engineered th e ca m chain system . W here the stock GSIOOO uses on ly sli ppers fo r tension . the modified version uses two sets of rollers a long wit h shortened slippers. The to p roll er from th e GS7 50 is mount ed in the stock GS750 loca tion . T o m inimize friction and reduce slippe r wear. the ca m chain is highly pol ished . Finally , the automatic tensio ner is m od ified to work manually . Feeding that fancy headwork is a set of very valuable "stoc k" carburetors. T he AMA rule for 1978 (ru les seem to change every yea r), required th e use of stock carb bodies. Each carb had to be disassembled and chucked into a lathe to be machined out to accept the " sm oo t h- b o r e " block. The real problem was that two out of every three ca rb bodies weren't thick enough and had to be discarded aft er the cutting tool open ed up the sid es of th e carbs. It took Pops two weeks of painstaking hand-fitting to produce a set of carbs for Wes' bik e . For 1979 , th e motor will breathe through Sl mm Ke ihin CR carburetors. Ignition is handled by a magneto CDI m anufactured by Kokusan Denki . The m agn eti c picku ps are mounted on th e right sid e of th e mot or where th e poi nts normall y go . T he ge ne ra ting system is plac ed on th e left sid e whe re the a lterna tor was origi nally. The left cra nk flywheel was excha nge d for one with a shorter sno ut to accommoda te the ignit ion . Timing is 12° sta tic a nd advances to 37° a t full. The 944c c GS750s that Yoshimu ra ca m paigned in 1977 wer e p lagued with cl utch problems. T h eir 130 ho rsepo wer wou ld ri p th e spl ines out of th e stoc k a lumi nu m inner hubs. T o co pe with the power, Suzuki provid ed the m with inne r hubs made of steel. The outer cl utch baskets ha ve spri ngs incorporated to a bsor b th e sh ock loadings of th e motor. These sp rings ha ve to be replaced wit h hea vy-duty items . The clutch pressure springs are replaced with Yoshimura units which a re made of valve spri ng material. The composition of the me.!al in the springs is of higher quality and ca p a ble of retain ing tensile strength much longer than normal clutch springs. Clutch lever pull is moderate considering the amount of power transmitted. Suzuki manufactures the close -ratio transmissions to Pops' specifications. All of th e engagement dogs are dovetailed, which causes the gears to pull th emselves together under torque. Aside from the ratios, th e main difference from th e sta nd a rd trans is that the teeth are precision ground rather than stamped . Prec is ion grinding gives smoother teeth and therefore less fr ict ion and more