Cycle News - Archive Issues - 1990's

TECHNICAL Randy Renfrow's AMA Pro Twins Grand Prix title-winner I n what was essentially its first full year on the I\MA Na tional road racing circuit, the H on da RS750 dirt track en gine- based Commonwea lth Racing Honda did what many considered to be T he Impossibl e: This one-off special won the AMA Pro T wins Grand Prix ti tle, beating the semi-factory Fast By Ferracci-bui ltl Dal e Quarterley-ridden Du cati 85!. Bu t the story really bega n some five years ago, when Kentucky resident Martin Ada ms began tryi ng to bu y an RS750 engi ne fro m American H onda. At the time, Ho nda wanted every one of the RS750 motors they bui lt to be run on dirt tra cks, for they were attempti ng to do minate the AMA G rand Nationa l Champions hip Series. By 1986, a cha ng e in ma nagement had led to a chan ge in heart , and Ada ms was fina lly a llo wed to purchase a motor. Faced with a clean sheet of paper, Ada ms decided to mimic th e best a 500cc Honda Grand Prix racer. T he front and rear ends of an NS500 were packed up wit h the engine and sent off to john Caffrey, a chassis builder in Wales, who was asked to ma ke the bik e as close as possible to a G P racer. Mea nwhile, ba ck in the ' States, Gordon Saund ers modified the fiber glass bodywork and aluminum gas tank off an '86 Honda RS250 to fit the bike. T he RS750 eng ine is, in fact , a mod ified mill fro m th e European Par is-Da kar repl ica bikes. T he poD bikes are sha ft-driven five-speeds, but for dirt track use the engi nes were converted to chain drive by adding a jackshaft to cha nge the rotation al directio n of the outpu t shaft. One gear was also removed from both the input an d o utpu t sha fts, as four cogs were deemed plenty for a dirt tracker. Oth er than that, the RS750 racers retain most of the street bik e's features: An offset, plain-bearin g cra nkshaft whi ch makes the engine run like a 9O-degree mill even though the air -cooled cylinders are splayed by only 45 degrees; a single chain-dr iven cam in each head operating four valves per cylinder with an incl uded angle of 38 degrees; and the sta ndard wet clutch. As delivered, the dirt track engines run 12.2:1 compression ratio pi stons in Nikasil liner s, an d produce 93 horsepower at th e rear wheel. Except for the titanium rods made by jet in so uthe rn California, th e entire RS750 was made in japan. In all, some 50 of these engines were prod uced, altho ug h a few ha ve since bi tten th e du st. All the pieces for the engine are available, so it is conceivable (albeit very expensive) that someone cou ld build an engine from part s. Back in 1984, Bubba Shobert had asked for a different carb layout on the front cylinder of hi s RS750 for more leg cl ea ra n ce. To have th e sa me downdraft angle of the ports and car bs, the front carb rod e higher than th e rear carbo So, in order to meet Bubba's req uest, a pair of different heads were produced with a lower port angle (cente rline 1.25 inches lower) and a less severe carb angle. 'Although the bike was then easier to ride, the lower port an gle cost about 1.5 hp, so the heads stayed on the parts room shelf. Fitting a large carb in the tight confines of a road racing fairing while having enough room for the rider's . knee seemed impossible, and Adams saw electronic fuel injection, with its compact throttle bodies, as the solution. Micro-Dynamics in England, an automotive electronics specialist, was ap proached to build an EFI system for the bike. At the time they were developing a fuel-injection system for the The uncommon Commonwealth Honda By Armen Amirian Photos by Brian Catt erson Quantel Cosworth, and exp ressed an ' in terest in becoming more involved with motor cycles. Dress rehearsal Adam s christe ned h is effort " Common wealth Racin g," a name derived from the fact th at Kent ucky is tech nicall y a commo nwealth, not a sta te, and th at Adams believes racing brings people togeth er. Thus, th e " Co mmonweal th Honda" was started for the first tim e in th e fall of 1986, and was ready for Daytona in Mar ch '8 7, wh ere despite sho wing much pro mi se, it would not run cleanl y in the race. Adam s asked T eam H on da 's engine man , Ray Plumb, to ta ke a look at the sick machine, and Plumb's first suggesti on was to switch th e bike to carburetors - a kn own quantity - .to simpl ify tro ubleshooting . A set of 41.5mm Mikuni fia t-slides were fitted , but the probl em s persisted. A complete engine teardown revealed tha t one rocker had fractured - proba b ly during dyno testing. Still, it was not a bad start for a one-of-a-kind bik e. . Repl a cing the EFI wi th ca r bs unearthed the old bugaboo o f the front cylinder's protruding car bo Because Plumb had worked with the dirt trackers, he kn ew about the low-a ngle front cy li nder heads. Honda lent Adams the only two low- an gle heads in existence to try on his racer, and it was decided that the horsepower loss was offset by th e convenien ce offered. After the new head had proven itself, Adams called Honda and asked if the low-port head pa tterns still exis ted. T hey did, and Hon da cast an additio na l two h eads especially for him. Dayton a '88 saw Australi an Paul Lewis lead -the Pro Twins GP final until a motor failure forced him ou t of the race near the end. Originally, the dr y-sump motor carrie d its 30wei ght Bel-Ra y oil in th e fram e backbon e. However, heavy braking a t Dayton a forced the oil forw ard , whi ch left th e rea r-mou nted drain h o le uncovered and the pl ain-bearing cra n k .gave up the ghos t sho rtly thereafter. A four -q ua rt vertical oil tank was ins talled beh ind the motor to cure the p robl em. La ter tha t year, di rt tracker Doug Ch andler rode th e bik e to its first win at Mid -Ohio, and Lewis backed that up with a close second-p lace finis h beh ind Qu arterl ey's Du ca ti at Laguna Seca. In th e final a na lysis. th ere was nothing wrong with th e fuel inj ection system . Ely Schi ess bought the system , and has been runnin g it successfu lly on hi s " Duco nda" (a Du cati Pantah bottom end fitt ed with Honda R?VC four-valve heads - not to be confused with th e origi na l, and similar, T wo Brothers Ra cin g " Du -d a " ). Even thou gh th e injection was not a t fault, it was decided to keep usin g carbs to simp lify matt ers. Although th e TM se ries Mikunis were o rig i na lly

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