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/127208
By Nate Rauba Photos by Kinney Jones hen Mr. T . Shif:en oya returned to Amencan Suzuki a s executive vic e president, motorcycle division, in 1984 from o ther assign ments within the co mpany, he brought back with him a new co mmit ment to racinga nd winn in g. Sh igen o ya held a similar posit ion in th e U.S: whe n Suzuki dominated in th e lat e '70 's and earl y 80's, th e Mark Barnet t! Kent Howerton era. Since th en th e Su zu ki RM motocross ers fell sligh tly behind th e competition , but wi th th e return of Shigenoya th ey'v e been st eadil y cl imb ing back to wards th e top. S h ige noya wa s i n fl ue n ti a l in bringing Bob H annah to Suzuki in 1987 to help develop th e RM line, and Hannah's influence o n th e 1990 RMI25 and especi ally th e 250 are evide nt. . While both bikes are unchanged cos me tica lly, there a re numerous internal changes that mak e th e new RM s work better, a n d be mo re W n ickel ; ~i l icone a nd phospho rous whi ch helps reduce wear. Cylinder porting has been alte red sligh tly on both sizes, with th e 125 ge tti ng resha ped exha ust ports a nd th e 250's rear tr an sfe r ports ' ra ised and en larged. Suzuki ' s Automati c Exh au st T iming Control (AET C) exha ust power-valve system is still retained with the valv es covering an add ition two millimeters a t low rpm in both engines to provide bett er lowend power. Both RM s received simi la r cy l in der h ead m odifi cati on s to resh ape the combustion cha mber for better power a nd combus tio n. On th e 125 the head gas ket has been replaced wi th a n O -ring, which is designed fo r bett er heat transfer betw een th e cylinder a nd head . It also increases com pressio n. Air/ fue l m ixtures reach both mot ors through Mikuni TMX carburetors, 35m m on th e 12S a nd 38m m o n th e 250 , th at brea th e through hew airboxes which have large air inlet s to p ro vid e mo re airflow. T he holes are a t th e bott om of th e airbox and cou ld easi ly ta ke resis tan t to wear, than last year's model s. Suzuki did th eir homework and made th e cha nges to co rrect th e flaw s in th e '89 RMI 25 and 250. Su zu ki's co ntingency p ro g ram also shows the com pa ny is co mmitted to racing as th ey've increased the money up for grabs to $2.8 million. Tech inspection Severa l of the mod ifications ma de to th e RM line are found o n both the RMI25 and 250, both with in the motor and o n the chass is. A new p iston, fea turing molte n pl asma injectio n, has been added to both models. Molten molybdenum is sprayed on th e wri st pin boss area of th e pi ston to reduce wear caused by side thrust of the co nnecting rod since late ral movement of the rod is now regula ted atthe small (p iston) end of th e rod in stead of a t the cra n k. T h is elim ina tes the need for thrust . washers which improves lubricati on of the big- end bearing. A thicker ring for better sealing was also added to th e RM12 5. Both models hav e a Suzuki Composite, El ecro-Chemical Material (SCEM) cylinder coa ting mad e of ' . in water, but Suzuki engineers pla ced a baffle in front of th e air filt er whi ch chan nels wat er and debris to large drain holes in the bottom . There is also a new cover for th e top of th e airbo x. The RM 125's inta ke tract is further improved with a 10mm sho rt er intak e manifold and reed-valv e modifi cati ons. T he reed -va lve stop per has been resh aped and the reed-valves a re thinner for better Iuel / ai r flow a nd improved midran ge p erformance. Th e 125 's exhaust pipe al so has add it ional gro u n d clea ra nce a nd mounting point to help prevent smashed or bent pipes. Suzuki fin all y added removabl e clutch covers to th e RMs to make clutch se rvice easier. The water pump is now a separa te unit, and moved farther forwa rd to help clear the rider's boots, so clutch changes won 't require a coo la nt cha nge. Needl e bearings were added to the clutch release p in ion for better feel o n th e 125, bringing it up to date with th e 250, which has new fibertype clutch dr ive pl at e mat erial. T here were no cha nges to th e l 25's six-spee d transmission, but th e 250 recei ved a new second gea r ratio for a better drive o u t of turns. Bob H annah d idn 't lik e th e way last yea r 's fiv e-speed tran smissio n shi fted, so th is year th e gear sh ift cam stopper has more preload and th ere is more space betw een the gear dogs. Neither of the ch rome moly, sem idouble cra dle frames were altered from last year. But th e 125 now has 4lmm inverted Kayab a forks, just like the 250. The low speed co mpressio n and reb ound damping rat es hav e been cha nged, a nd o il flow th rough th e ca r tr id ge sy st em improved. Oil now recirculates in the fork s whi ch reduces the cha nce of air mixing with th e o il. There are now 21 rebo u nd and , compressio n damping adjus tme nt s on both models, and a n easy-to -use bleeder screw a t th e top of th e for ks. At th e rear end the shocks have li ght er low -speed co m p re ss io n da mping, and th e shoc k sha ft di am eters ha ve been in creased from 14 to l6mm for in creased strength. Both models hav e 18 co mpression a nd 21 rebound damping adj ustments [or fin e tuning to track conditions. T he RM125 also ha s new FullFloater suspe ns ion linkage rati os wh ich a re more progressive to prevent shoc k bottoming. Suzuki made the swi tch to 19-in ch rear wh eels th is year for the tracti on Although they look almost identical to last year's models. the 1990 RM125 (left) and RM250 (right) feature significant internal changes.