Cycle News - Archive Issues - 1980's

Bolt-on towers cover each set of valves and carry tappet buckets, dealing with side loads generated by offset rocker arms. (Above) Graham's bike without its exhaust pipes. Oil tank fits ahead of rear wheel. (Below) External braided steel lines feed oil to the top end: (Above left) The crank pins are offset 45°; (above right) cast ART pistons are nearly flat; (below) forged titanium rods come from Crower. To avoid that problem, Honda engineers came up with a novel solution. After the valves. springs, retainers and keepers are in place, an aluminum alloy LOwer bolts over each set of valves. Each LOwer has two bores, one centered over each valve, and tappet buckets fit into those bores. Tbe arrangement is similar to standard DOHC valve gear practice, except shims aren't used to adj ust lash. At any rate, the large-diameter tower bores and tappet buckets can easily deal with side-loading from the rocker arms, and the valve stems are pushed . straight down, without side loading. The bronze valve guides are trimmed flush with the ports to facilitate air flow; the ports themselves consist of two subports (one for each valve) blending together just before the intake manifold and the exhaust head pipe. The valves are made of titanium, the intakes measuring 30.5mm, the exhausts measuring 27mm. Intake valve timing is 45°.61°, exhaust valve timing 60.5°-43°, measured to and from lmm lift. Valve lift is9.8mm for both intakes and exhausts. Epoxy paint coats the inside of each head's valve gear compartment, eliminating any chance of an oil leak due LO a porous casting. Valve angle is 19°· from vertical (included angle is 38°) and the valves are packed tightly into the shallow combustion chambers, with a centrally-located spark plug. Valve seats are bronze. Compression ratio is 12: I. The pistons are ART castings, flatLOpped, with three rings. Each aluminum alloy cylinder has a Nikasil liner: a nickle matrix with about 5% sil icon carbide particles is bonded to the aluminum surface, replacing a conventional pressed or cast-in iron liner and producing a lighter cylinder with beller heat-transfer characterislics. The engines are built by Honda RSC in Japan and shipped to the U.S. Engine maintenance, building and R&D is handled by two men, Rob Muzzy and Ray Plum. The engine is housed in a C&J frame, which is on sale to anybody who wants one. Rake is adjustable by installing different bearing cups; the frame comes set up at 25.5° and cups are available to set rake at 24°, 24.5°, 25°,26° and 26.5°. Each degree of rake change alters wheelbase about ~ of an inch; wheelbase with 25.5° is 54.5 inches. The frame, including swingarm and'mounts, sells for $1095. C&J also sells the seat and tailseetion, for $135. That's the easy part. The catch is that C&J doesn't include the RS750D's complicated rear shock linkage with the frame. That linkage connects a Showa shock to the swingarm and frame, and is designed LO make the RS750D's suspension linear, not progressive. It's no secret why Honda wanted non-progressive suspension: an XR750's rear shocks are mounted at the rear axle, nearly vertical, yielding a 1.7% rising rate suspension: that's essentially liner. Building a linkage to duplicate the RS750D's would cost an aspiring privateer close to $1000, according to fabrication experts, and the Showa shock is nOI available at any price. But Works Performance will cusLOrn-build shock absorbers LO work on an RS750D frame and swingarm- without a linkage, yielding progreso sive (rising rate) suspension. Such a custom·built shock would sell for around $450. The Showa forks aren't available, either: they're based on the Honda Superbike forks, with 4l.3mm stanchion tubes and externally-adjustable damping. Anybody who buys an RS750D engine from American Honda (the price is $8500) gets a list of sources for wheels, chassis parts, oil tank, and exhaust system, along with a service manual, a parts list, a parts price list and parts ordering information. More information is available from • Keith Reeves at 213/321-8680. Specifications Engine type DOHC air-cooled four-stroke V-Twin. Displacement 749.5cc. Bore x stroke 79.5 X 75.5mm, Compression ratio '12: 1 . Carburetion , (2) 38mm Keihin. Gear ratios Primary _. , 2.348. Secondary 1.031. .. 4th 1.000. 3rd 1.154. 2nd 1.391. 1st · 2.056. Fuel capacity 2.6 gal. Wheelbase 54.5 inches. Rake 25.5°. Trail 3.6 inches. Redline 9500 rpm. Valves per cylinder 4. Intake valve size 30.5mm. Exhaust valve size 27mm. Intake lift 9.8mm. Exhaust lift 9.8mm. Included valve angle 38°. Intake valve timing 450-61 0. Exhaust valve timing 60.5°-43°. (measuring pt.) 1 mm. Horsepower. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 93 bhp @ 8000 rpm. Torque 61.2 lb.-ft. @ 7000 rpm. 19

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