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
‘THE CRANKSHAFT IS IN THE UPPER PART OF THE CASE, THE TRANSMISSION IS BENEATH IT’
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