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/126005
; .. . 11III ~ t ~ r-, O"l ....-I N I-< llJ ..0 8 llJ ..... 0.. llJ en Cause ~nd effect. RM cylinder is on left. with four transfers. 250 jug is more radical . Note internally X-braced piston. reed valve b~dY bolts to cylinder base. feeds crankcase dirt. ", . A TECH BRIEFING SUZUKI'S LONG-LIMBED, LONG-WINDED RACERS . By Lane Campbell The man IS saying, "This IS the closest thing we've ever had to works machinery," and we're all yeah, sitting around nodding where have we heard this before. But close inspection and a day's track session is enough to convinc e even the skeptics. Aside f r o m one or two d etails, the just-released RM 250 and RM 370 motocrossers are the works machines of two seasons past tooled up for production . I c an remem be r s crut inizing DeC oster's prac tice 370 a couple days before las t year's Trans-AMA and wonder ing why the extra p air of cylinder base stu ds at th e rear of the cylinder. Now it's out in the open ,- a two-petal stainless steel reed bolts to the rear cylinder base surface , pointing straight into the cran kcase. It works in conjunction with a normal pis ton port, opening to satisfy the engine's demand only at high speed. What 's the point of this unorthodox reed? You have to isolate the intake system of a t wo-stro ke in your mind a minute and consider what 's happening at various engine speeds. A piston port is simple and non-obstructive, but it has a co up le of glaring di sadvan tages. It forces the "i n t ake op en " period to be symmetrical about top dead cen te r, while the engine's "demand" for air, as determined by crankcas e pressure, is non-symmetrical. In take port ti m ing has to be at best a bad compromise between low and high speed efficiency, with optimum crankcase charging in only one narrow RPM band . A convent ion al reed is a n engine-controlled demand valve , and tends to y ield optimum cran kcase charging over the entire RPM band, though it is far more restrictive throughout. Piston ports yield peak power, conven tional reeds yield a broad powerband . .. Suzuki 's " p o wer reed" is an attempt to have the best of both worlds. They go with a rel atively " mild " piston port, then when the engine begins to " ru n out of wind," the reed opens up _ On both 250 and 370 engines, th is occurs near 50 00 RPM. Now we begin to bring th e intake system into harmony with the rest of the engine. We find that the RM 250 uses six transfers that wrap fully around the intake, much as did the RM 125. The 370, however, has fo ur transfers (lacking what we'd call the "booster" port) and is, overall, more mildly ported than the 250 (see spec.). The intake on th e 3 70 is bridged for more piston support. One of the ques tions th at ari ses is " co m m onality" - wh at will re trofit to th e TM series and what parts . are com m on between the two new bikes? As to retrofi t - nothing. En gine s and ch assis are ne w from the ground up ; converting a TM to RlVI spec is virtually imp ossible. The new bikes use an identical ch assis, but the en gines are totally different. The 370 engine is a long stroker and slightly undersquare (77 x 80 mm ), whil e th e 2 5 0 engine is more c o n ve n t i o nally overs q u are (70 x 64mm) . The humongous stroke makes th e entire 370 en gine a significantly RM250-370 chassis \- { - 8.1/3" -r 4.96" Port timi ng - in crankshaft degrees Intake Transfer Exhaust RM250 78.9 63 95 meas. from TOC meas. from SOC meas. from SOC RM370 72 67 96 Compression by-pass hole RM370 cylinder taller unit, a fact which gives,ea ch bike a distinct handling personality of its own, even though the frames are iden tic al and the dry weigh ts are within 11 pounds. The 370 carries its en gine weight so much higher in the chassis that it literally feels like a bigger bike. As mentioned before, each en gine is a unique design unto itself and new from the crank (full-circle flywheels with a pi ston-centered rod) outward. Putting the rod-locating spacers at th e small end allows more clearance for lubrication at the crankpin. The piston is drilled to oil the exhaust port bridge and is X·braced internally. The 370 piston uses a keystone ring, the 250 uses a 1.04mm straigh t thin ring. Ignition is COl with redesigned rotor. The " black bo x" resides behind the number plate up fran r. Gone forever is th e oil pump. Fuel is strictly 20: 1 premix, and th e fac tory reps say no ch eating - period. Both en gin es h ave gear primary drive, but for. structural reasons, the 370 does not h ave primary kickstart. The 250 does. . Both heads are a "squash-dome " design to induce turbulance in the fuel-air ch arge. The 370 again is differen t , h av in g a modi fied leading-edge squash area. The 370 also h as a trick for easy star t ing - a tiny cap illary bypass hole ab ove the exhaust port that bleeds off some co mp ression at low cranking speeds. Both engines use a 36mm Mikuni with only je t ting charges between them . The factory 's rationale is, why not tune th e 370 mild - it already makes more horsepower than most people can h an dl e. After riding it , none of our te sters could fault their reasoning. The ch assis that both machines share is "state-of-the-art." for curren t works machinery : laydown Kay ab a gas shocks and leading-axle forks combine to give eight inches-plus of effective travel at either end. The general layout is similar to that of the RM 125, only the dimensions have been changed to protect the innocent. The s hocks are of the same free-piston design as th ose on the RM 125. The free piston separates the gas, pressurized to 220 psi , from the shock fl u i d . Th e shocks are sealed, non-rebuildable, therefore the damping is se t. Th e shocks carry a twin-element spring with rates of 77/233 pounts/inch. Other rates and combinations can be had. Prel o ad i s a d j u s t a ble by repositioning a bottom spring clip. The fo rks allow more room to play around with spring and damping rates, and our . te st riders' impressions lead us to believe th at having the forks mismatched to the rider tends to do weird things to the rear end's behavior as well. When dialing an RM in, it appears best to start at the front and get that right first. . The shocks are designed with a quick-acting popoff valve in the piston th at allows free , relatively . undamped re sponse to small, high-frequency stutterbumps, Judginjt from the forks ' behavior under the same kind of stress (t hey tended to pack down and hydraulic -lo ck) this same design philosophy should possibly be applied to them as well. Light alloy is everywhere. Magnesium in the engine cases, hubs and backing pl ates, fork sliders o f "DuraIumin" alloy so they ' can be cast in a thinner section, alloy tank and air box as well. In summation, the new RMs are so trick out of the box, they boggle the ' mind. They'd need only rem ote shock oil reservoirs and different ignition to be true " replicas " of previous works bikes. For Suz uki (and the other Japanese firms) that in itself is q ui te a turnabout, (More photos and spec on next page) - ..