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/126521
.-4 00 0') .-4 IAbovel Side vlewa lhow'the engine .. e pertl8lly-strelMd member of the fnIme. with downtubel connecting to the engine In the mlcklle of the V. IBelow IeftJ Mechlned e!umlnum left IkIec:ue replecu Itock ClIH. eltemetor end ItoCk Ignition. IBeiow cent8rI RIght IIde IhowIItoCk c:aee. DeIIorto 38mm cerb. end pelnt end grephlCl work by MoIly.IBelow right) RIIce In eetlon. IBottom right) Freme mount et front cylnder. CIOSQ-Up ThoYamaha 750V-twin dirt traekofj By Dale Brown When Rookie Expert Jimmy Filice qualified his Yamaha Virago- based Roberts/Lawwill Racing machine into the field at the San 10 Jose Mile, it was still only another step in a crash course of producing a competitive racer out of a brand new engine. Mcrt LawwiU is contracted to do rescarch and development for Yamaha on both the 750 V-twin and the soo sin· gle. In the middle of January Lawwill picked up the first chain drive engine to arrive in the U.S. The ensuing months saw lots of work as they sct out to produce a racing engine out of a brand new street motor. "Nothing had been done before," explained Lawwill. "We had to produce our own blanlts for pistons, the same for the cams;, Each step eats up a few more days. The engine that appeared at San Jose looks fairly stock, and in fact the transmission and lower end are stock. The left sidecasc is machined aluminum, and fits where the alterna· tor and stock ignition used to reside, making for a narrow motor. Ignition is via a dual points system mounted on the front cylmdcr that fires two coils. Left side shifting is retained, but the motor runs in one gear around the track anyway. On the right front of the motor, an aluminum plug is fitted where the staner motor used to be. The heads were flowed and reworked by C.R. Axtell. They're fed by 58mm .oeUorto carburetors, and the intake manifolds havc been altered for better position of the carha. Forged pistons and cams, both of the group's original design, rcsidr within tbe motor. About the horsepower the motor puts out, La_ill is optimistic. No dyno readings were given, but Men said, "It took us five years to get as much horsepower out of the Harleys as we have now with the Yamaha." The "boom box" requirements for the San Jose Mile - a race under pres- sure from nearby residents - hun the Yamaha. With the boom box, they were unable to get equal length on both headers, becausc the rear cyIin· der's exhaust port faces to the rear, and there's not that much room between the rear of the engine and the tire. The blue frame stands out on the machine. Built by Dick Mann and La_ill up in nonhern California, it utilizes the engine as a partiaUy-stresIIcd member of the frame. In the original Virago application, the engine was connected to the frame front and rear. In the Roberts/La_ill application, additional frame tubes come across the middle of the engine, connecting it to the frame at the top of the front cyl. inder, the middle of the V, and at the rear of the motor. The frame itself is chromoly. Wheelbase is a fairly standard 56·57 inches. depending on chain adjustment. Suspension units are by Simons (front) and Works Pmonnance (rear). Both manufacturers arc working c10scly with the Robcrts/Lawwill group to perfect the units for this application. Track time at San Jose provided some valuable feedback. Lawwill plans changes in frame pmetry, suspension and the en~ne's power curve to begin with. Additional development will be carried out in coming weeks. Once they havc a working machine, Lawwill plans to build for saIc 25 rolling chassis, incorporating everything they've learned. And just about everything they've pumped into the bike will be for sale, including cams, pistons, the sidecase and other pans. A lot of hours were turned in on this project, with the sccond of the two bikes being finished only hours before San Jose. Joining LawwilJ in the effort were Axtell, Mann, Mike Libby. Bud Aksland, Parra Osiocain and Denis Jones. San Josc's appearance of "a Rookie Expert on a rookie motorcycle" was just the beginning...