Classic Bike (UK) - - Classic Workshop -

Be­fore the Gold Wing, all Honda mo­tor­cy­cle en­gines were air­cooled. Wa­ter-cool­ing the flat four avoided over­heat­ing of the rear cylin­ders, masked from the air stream by the front pair. It also muf­fled com­bus­tion and me­chan­i­cal noises to achieve ex­cep­tion­ally quiet run­ning. In ad­di­tion, once warmed up, wa­ter-cooled en­gines run at near-con­stant tem­per­a­ture, al­low­ing for closer in­ter­nal tol­er­ances to pro­vide con­sis­tent power out­put and re­duce hy­dro­car­bon emis­sions. The draw­backs are more weight and the added com­plex­ity of a wa­ter pump, a ther­mo­stat and a bulky ra­di­a­tor with an elec­tric fan to cool it at a stand­still. alu­minium el­bowed in­let tracts. Each head is held to the crank­case casting by six 12mm bolts and topped off by a bolted-on cam cover.

Four con­stant-vac­uum Kei­hin car­bu­ret­tors are fed fuel from the un­der­seat tank by a me­chan­i­cal pump driven off the rear end of the right-side camshaft. Link­ages con­nect the push-and-pull throt­tle ca­ble to the four but­ter­fly throt­tles and chokes from a sin­gle con­trol ca­ble. The bod­ies bolt up to a two-piece al­loy air box with mesh dis­pers­ing screens over each pair of in­takes. The front-right carb has a cut-off valve to reg­u­late the pi­lot air sys­tem. The valve closes to richen mix­ture and pre­vent back­fir­ing when the throt­tles are shut at high rpm. A large can­is­ter con­tain­ing a re­place­able fil­ter el­e­ment fits on the open top of the air box.

There is a fly­wheel on the crank­shaft be­hind the rear­most plain bear­ing journal and be­hind that pri­mary drive is taken down to the gear­box main­shaft in the right-side crank­case by a HY-VO chain. Re­strained on both runs by guides faced with syn­thetic ma­te­rial, the chain is lu­bri­cated by an oil jet, and a steel trough around its lower run con­tains splash. The driven sprocket has a built-in cush-drive as­sem­bly with rub­ber seg­ments to smooth power de­liv­ery and two nee­dle roller bear­ings al­low it to ro­tate in­de­pen­dently of the shaft, which is sup­ported in the crank­case on ball bear­ings, as are all the trans­mis­sion shafts. The sprocket has a sleeve sec­tion that projects through the rear bear­ing and car­ries the con­ven­tional six-spring mul­ti­plate clutch as­sem­bly on splines. The clutch’s al­loy cen­tre drum is splined onto the main­shaft and the bas­ket (an al­loy plate holder braced by a steel band riv­eted to a steel cen­tre) is re­tained on the sleeve by a cir­clip. A du­plex sprocket on the back of the bas­ket cen­tre car­ries a chain driv­ing the oil pumps in the left-side crank­case. The ca­ble-op­er­ated ball-an­dramp clutch re­lease mech­a­nism is mounted in a cir­cu­lar casting fixed to the rear en­gine cover, with its own smaller ac­cess cover.

The gear­box se­condary shaft is to the right of the main­shaft and, like all the trans­mis­sion shafts, sup­ported in ball­races – some with grooves on their outer cas­ings for C-rings to lo­cate them lat­er­ally. A slot­ted drum ac­tu­at­ing three se­lec­tor forks is un­der­neath the gear clus­ters. A fork on the end of the shaft that car­ries the gearchange pedal trans­mits ac­tion through 90° by

en­gag­ing with an arm pinned to a shaft run­ning through the left crank­case into a com­part­ment in­side the front en­gine cover. Here a link­age and spring-loaded pos­i­tive-stop mech­a­nism en­gage with four rollers fixed at the front end of the drum. A large gear on splines at the rear end of the se­condary shaft trans­mits drive to a slightly smaller gear on the out­put shaft, sup­ported on ball­races in the right-side crank­case and rear cover. In­cor­po­rat­ing a spring-loaded face-cam damper, the out­put shaft con­nects with the fi­nal drive shaft to the rear wheel via a uni­ver­sal joint.

A large gear at the rear end of the crank­shaft is driven by the starter shaft to the left of it, which is sup­ported on two ball­races, the larger rear one be­ing held in a mount fixed to the rear wall of the crank­case. The shaft has two driven gears that are part of a shock-ab­sorber mech­a­nism con­tain­ing two sets of rub­ber seg­ments and vanes, plus a coil spring con­cen­tric with the shaft. The rear of the shaft pro­trudes rear­wards through the large bear­ing to carry a three-roller clutch mech­a­nism run­ning on a nee­dle-roller bear­ing and driven by chain from the starter mo­tor, which is at­tached to a for­ward-fac­ing flange on the rear cover. On the end of the shaft is the heavy (4kg) al­ter­na­tor ro­tor, which helps coun­ter­bal­ance crank­shaft forces by ro­tat­ing in the op­po­site di­rec­tion. It sur­rounds an 18-coil sta­tor fixed in the up­per left por­tion of the rear cover.

En­gines from the first three years of pro­duc­tion have a kick­start mech­a­nism with a de­tach­able pedal, mainly for turn­ing the en­gine over dur­ing ser­vice op­er­a­tions. When the pedal is re­moved, a wa­ter­proof cover fits over the end of the stub lever it slots into. A shaft with a con­cen­tric re­turn spring is mounted in a flange on the back cover and en­gages with the rear end of the crank­shaft via a ratchet. There are two gero­tor-type oil pumps in the left crank­case, turned by a com­mon shaft with the driven sprocket bolted onto its rear end. The larger pump near the front of the crank­case draws oil through a re­mov­able strainer unit in the wet sump and sends it through a car­tridge fil­ter mounted on


Wa­ter cav­i­ties sur­round each linered cylin­der bore

Each big-end bear­ing has its own journal on the crank­shaft HY-VO chains have been around since the 19th cen­tury

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