Cycle News - Archive Issues - 1980's

But because three valves are lifted rather than two, and because three valves have more total circumference than two, the actual intake area around the valve's head is increased by more than 10%. This increase in effective air intake improves both horsepower and torque by about 10% and makes the torque band much wider. Valve adjustment is by shims on the inside of the lifters rather than by pads set in the ends. This permits a wide.r cam-lobe contact area and reduces wear because the pressure/area ratio is lower. To facilitate valve clearance inspection (inspection can be made in the same manner as with conventional designs), the cylinder' head is split horizontally into two major parts. The upper section is the camshaft case; the lower contains valve gear and combustion chambers. The camshafts and their case are removed to shim the lifters. Separating the camshaft case from the main cylinder head ~eans that the positioning of valve lifter geilr is not dictated by the locations of the cylinder head hold-down bolts. This permits a much more compact power unit. The head and cylinders can be removed without taking the engine out of the frame - another plus. Extensive research was done to find the ideal materials for cams, tappets, shims and valve seats. Unlike conventional cams which are· hardened only on the lobe lifting surfa- . ces, the entire circumference of the FZ750 cam lobes have been carburized. With a hardness of HrC 48-52, they are much harder than the HrC 45-48 ofconventional cams. In addition, the carburized lobe surfaces retain oil better for improved lubrica- tion. Lifter crowns are of a sintered metal material. The valve seats are of the highest grade sintered iron and copper. Yamaha tests have shown that following the initial valve adjustment after break-in, valve clearances will remain virtually unchanged. The cams are driven by a lightweight, competition-quality bushed chain, with automatic tensioner for increased reliability at high rpm. Central support cam bearings are located in what is usually dead-space between lobes. Head width is 17mm narrower than the XJ750, due also of course to the closely-spaced cylinder bores. The cam case cover is made of rnagAesium to save weight. For instant throttle response, Mikunj BDS34 constant-velocity carburetors are used. First developed for the XVZI2, these downdraft carbs have special lightweight plasticslides and offer very little flow resistance. With the added space.available behind. the steering head. they can be mounted i.n such a way that all intake tracts are straight and of the same length unlike those on conventional inlinefours where the outside tracts must angle inward. Four 3.9-inch-long air horns run from the carburetors into the airbox and assist intake inertia and pulsation. Intake temperature is crucial for four-stroke engine performance. Generally, the lower the temperature the better cylinder filling is achieved. With the airbox located above rather than behind the cylinders, the carburetors get a steady. undisturbed flow of cool air. An insulator between the head and carbs keeps carb temperatures down, and a heat shieldde£lects hot air from the radiator away from the carburetors and routes it out the Yamaha engineers used this single-cylinder engine to test the five-valves before designing a complete new engine. p~r-cylinder concept fairing ducts. Intake tract temperature is further reduced by the engine's liquid cooling system. Exhaust performance is also improved because the slanted cylinders permit straighter exhaust ports and . the header pipes are less curved. Unlike a V-4. header pipe length is equal and exhaust pulsations can be used to speed the flow of pent gases. Bell mouths at the passageways inside of twin mufflers increase flow volumes by 20% over straight pipes and further accelerate exhaust flow. Slanting the cylinders forward allows the upper engine weight to be mounted further forward and down. This puts more weight on the front wheel and allows almost equal front/ rear weight distribution while at the same time lowering the center of gravity. With the carbs and airbox up front. the rear part of the fuel tank can be dropped down behind the cylinders. This lowers the center of gravity even further and concentrates weight mass around the center of the motorcycle. Unlike machines equipped with conventional fuel tanks, the center of mass drops a little vertically (Continued to page 80) ·YA AHA, Oh what fun i. is to ride! E.\ CIT 1: ~I I: :'\ T . S 1: .\ soN E.\C1TE~IE:'\T·SI' \sON Moto-4 Huge Inventory of A TV Accessories Team Yamaha Center 75

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