1979 Honda CBX Road Test - page 7

The camshafts are in the normal places. The two exhaust cams, like the two inlets, are joined together by an Oldham coupling. The longer the camshaft the more difficult it is to keep it true during the grinding and heat-treating procedures. The CBX's cams, were they onepiece, would be very long-thus the Oldhams. To avoid the ongoing problem of cam-chain whip caused by oscillations and torque reversals in the crank and the camshafts, Irimajiri decided to use two chains. They are 9.5mm Hy-Vos which, like the primary chain, are manufactured for Honda by Tsubakimoto under license from Morse. The exhaust cam is driven by the crankshaft through a dual sprocket bolted just outboard of the Oldham coupler. The inlet cam is driven by the exhaust cam through the second chain. There are two spring-loaded tensioners: one works against the rear run of the chain from the crank, the other against the lower run of the chain which joins the two cams.

The cams are hollow, made from cast iron; the couplers are nickel-chrome-molybdenum steel. The cams rotate directly on eight cast-in bosses in the head (no bearing inserts are used); the four outboard bearing surfaces and their caps are machined to engage discs on the cam ends to establish positive lateral location.

The inlet valve heads measure 25mm; the exhausts are 22mm in diameter, and the valve length is 89.7mm. The valve heads are stellite-faced, close against sintered iron valve seats and are controlled by conventional inner and outer valve springs which, like all Honda valve springs, are made by Nihon Hatsujo. Valve lash is set exactly the same way you'd see on a KZl000, or a Yamaha 1100, or a Suzuki GS750: with different thickness discs fitted to recesses atop the tappets. On the GP 297 Six, Irimajiri used Winkler caps which went between the underside of the tappet and the top of the valve stem, just like the Kawasaki KZ650Four. "That reduces valve assembly weight," Irimajiri said, "but makes maintenance difficult. We had lots of time between GPs to remove the camshafts and adjust the valves. But with four cams and two cam chains, it didn't make sense on the CBX." Surely he had heard about the Z-l's tendency to spit its shims through the cylinder head at high engine speeds? "That is caused more by valve float than anything else," he said. "Valve float can be restrained by using the lightest valve-train components possible, which was one of the reasons we have a four-valve engine. To see if we would have a problem in this area, we put an engine on the dyno and tested it to failure at 11,000 rpm. We had no problems with the valve train - a cylinder stud broke, after 30 minutes."

The CBX's valves are extremely long, just like the valves in every Honda ever built. The valve length was mandated by an ongoing desire for optimum inlet port angle and shape. The longer the valve the steeper the inlet port's angle of attack can be, and the steeper the angle, the better the flow past the inlet valve head. Inlets and exhausts are inclined 31.30 degrees from the centerline of the bore, for an included valve angle of just under 63 degrees. There are different valve angles for different engines, Irimajiri explained. A greater angle promotes better flow, because in a given engine a wider angle provides room for larger valves. But a narrower angle creates a more compact combustion chamber which, all other things being equal, can lead to more horsepower. In the most general of terms, he went on, valve angle depends on bore-stroke relationships. A short-stroke, wide-bore engine can use a narrow included angle, because the bore dimension provides enough combustion chamber room for acceptably large valves. But a long-stroke, narrow-bore engine would need a greater valve angle to achieve the optimum valve size. Too, he said, there are considerations of proper cylinder head cooling: the narrower the angle, the worse the cooling because there isn't as much room to circulate air across the top of the combustion chamber. "Many, many compromises," he concluded.

The underside of the cylinder head looks for all the world like six XL 250 heads welded together. Combustion chambers are of entirely conventional pentroof design, spark plugs occupy the chambers' dead-centers, and there are flat squish areas to the front and rear. What is unique - and certainly frustrating for Irimajiri - has to do with the wandering relationship between the inlet valve seats and inlet port centerlines. Ideally, the ports - and their carburetors - would be perfectly centered upstream of the inlet valves. That's true of cylinders three and four - the two closest to the middle of the engine - but less true of cylinders two and five, and not true at all of cylinders one and six. Again, this goes back to Otsuka's request for as little carburetor width as possible to provide adequate space for the rider's knees. The inlet ports therefore angle inward. They are also of different lengths: shortest towards the middle, longest out at the ends. This Irimajiri dealt with by filling different-length tubular extensions inside the plenum that feeds air to the carburetors, so as far as the ports are concerned, the distance from atmosphere to valve seat is the same for all cylinders. Even so, he says, some cylinders breathe better than others.

Like the combustion chambers, the pistons are entirely conventional: semi-slipper ART numbers, three rings, flat-topped crowns cut away for valve clearance. They attach to forged steel connecting rods with no small end bushings and ordinary plain bearing big-end inserts. The cylinder casting is a single piece with bore centers 82mm apart and replaceable cast iron liners. A total of 16 studs - of two diameters - secure the cylinder and head to the crankcase.

The crankshaft turns in seven main bearing saddles fitted with insert bearings made of the same material the rods use. Explaining that his biggest problem was controlling the extra-long crank's torsional vibrations, Irimajiri did not elaborate beyond stating that he "had had experience here before." Slightly off-center to the crank's right is the drive sprocket for the exhaust cam; off-center to the left is the massive Hy-Vo that carries power from the crankshaft to Irimajiri's beloved jackshaft, its bottom run partially obscured by a baffle plate and a hydraulic chain tensioner. Pressure for it is tapped directly off the oil pump. At normal engine temperatures the tensioner operates with a pressure of 71 pounds per square inch. Although Hy-Vos are not conventionally in need of tensioners of any kind, this one is necessary to keep the primary chain from rattling around.

The jackshaft - the single component which made it all possible - has a transistorized breakerless ignition fitted to its right end. There are three pulser coils and a timing rotor beneath the chrome outer housing cover; they work with three black boxes mounted deep below the seat, and three dual-lead coils hung under the fuel tank below the chassis' main backbone tube. The coils fire cylinders one and six together, three and four together, and two and five together.

Pointless ignition is now fitted to all high-performance Hondas. It is especially important to the Six, which is in an extremely potent state of tune. According to Irimajiri, over 50 per cent of all engine problems on the 750-series bikes were related directly to the ignition; he could not have that on the CBX, because a bothersome little problem on a tamer engine could amount to a major disaster on the hot-eyed Six. For starting and idling, ignition timing is ten degrees BTDC; at higher engine speeds it is electronically advanced to 42 degrees. The Six is strikingly free of traditional Honda cold-bloodedness. On the chilliest of Willow's chilly mornings it started instantly and was immediately ready for business. We do not know if the same will apply in North Dakota in February; we suspect it will.

Beyond the extent to which individual gears have been lightened, there is nothing apart from brawn to distinguish the CBX's transmission. Both shafts turn on balls or rollers. To quell noise and backlash caused by the gear pair driving the clutch, the drive gear is fitted with a spring-loaded auxiliary gear which carries one more tooth than the main gear. The two gears turn at different speeds, and hold the clutch gear in absolute engagement at all times.

The alternator is not directly linked to the jackshaft; instead, it is driven by a twoplate all-metal clutch, which shields the massive alternator rotor from the engine's frequently violent rates of acceleration and deceleration. The alternator clutch in the test CBX showed excessive wear; the composition of the metals used may be changed on the production bikes.

Irimajiri calls the CBX's carburetors "the most sophisticated ever used on a street-going motorcycle." They have 28mm venturis, are manufactured by Keihin, and were selected to smooth the engine's rate of acceleration. Two features make them unique: an accelerator pump attached to carb number 3 which shares the squirt with the other five through a gallery, and a deceleration, or air cut-off, valve attached to the number one carburetor which also communicates with the rest through a gallery. The purpose of the valve is to prevent backfiring on rapid engine deceleration. The valve in this system closes when intake vacuum reaches a specified level, which shuts off air flow through one of the low-speed jets and promotes a richer mixture.

We have never experienced CV carburetors that work as well as these. They're responsive, vernier-accurate, and they do exactly what Irimajiri says they do: assure smooth engine performance. It has long been claimed by manufacturers that CV carburetors' main reason for being is to isolate the engine from the often-indelicate fist gripping the throttle by allowing the carburetors to decide for themselves what's best for the engine under prevailing conditions. Until now just the opposite has been achieved; CV carburetors, especially those fitted to the larger Yamaha four-strokes, traditionally have made all the wrong decisions and in general have mucked things up. The CBX's, however, are perfect.

   

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