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Issue link: https://magazine.cyclenews.com/i/126783
, mc ine c ose to orizonta. It ItStwo crankshafts, the RZ500 is therefore quite different from a four-stroke V-motor and the V-4 two-stroke NSR500 Honda campaigned by Freddie Spencer this year. The '84 GP Honda uses one crank but its cylinders are offset; two cylinders lieforwardjust below horizontal while the other two cylinders are canted back 90°. As is necessary with a two-stroke motor, all four cylinders have their own crankcase chambers which are all fed individually via reed valves from a bank of four adjacent carburetors. In this way the NSR500 Honda is a new solution to an old problem while the tandem crank set-up of the Yamaha is a much more traditional design. The interesting and unusual thing about the RZ500 is the method of induction control. On each pair of cylinders it is different I The rear cylinders have reed-valve assisted piston-port induction with a quite conventional layout of transfer ports. There are four main transfer ports with a fifth transfer direct from the roof of the intake port. And to improve scavenging under the piston, the rear of the piston has two windows cut into it. But the set-up on the front cylinders has induction direct into the crankcase, again controlled by reed· valves, with the four-petal reed-blocks being visually similar to those of the rear cylinders. The use of these two different induction systems seems to be the result of a space problem. Between the narrow 50" displaced cylinders there is not a lot of room for the intake manifolds, but by using the two different - induction systems, all the necessary plumbing can be accommodated. The lack of space in this are-a and more specifically the lack of room for larger reed blocks would seem to be the limiting factor in the further development of the RZ500. At this juncture it is worth noting that the 1983 factory race bikes used rotary disc valve induction with a single, double cutaway disc controlling the induction for two cylinders. The discs were located in the V between the cylinders in the area conventional reed valves would be located, with the disc rotation axis at right angles to the crank rather than in line with it. This year, however, Eddie Lawson has fronted on the grid on several occasions with a bike using reed valve induction. At 56.4 x 50.0mm, the bore and stroke dimensions of the RZ500 are more oversquare than the 54 x 54mm dimensions of most road-going twostrokes with l25cc cylinders. These oversquaredimensions ha.e been used in the more recent TZ250s and in Suzuki's more recent 500 factory racers. Internally, the RZ500 is a conventional two-stroke with built-up ball and roller bearing cranks, two-ring pistons and forged steel rods. The four induction tracts are fed by 26mm slide throttle carburetors arranged so that each pair is on synchronized linkages. The carburetors sit just to the side of the V between the cylinders and are connected to their respective intake ports by right-angled manifolds. Two separate air boxes sit above each pair of carburetors and are mounted on the inside of the fairing. The compression ratio is 6: I measured from the closure of the exhaust port, the exhaust port timing beinji( varied by Yamaha's well-documented Power Valve system. Without going into too much detail, the exhaust port height, and thus, exhaust timing, is varied according to engine rpm. The valve closes at low rpm for tklayed exhaust timing and better (Above) A pair of RZ cylinder assemblies. with reed-va1ve and power-valve (left) removed. (Below) The right front and left rear pistons reach TOC at the same time in this view from the front. bottom-end power while at higher rpm the valve opens to optimize high rpm running. The valve itself, or more correctly the two valves, are operated by an electric servo motor via a push/pull cable. Primary drive from both cranks goes direct to the clutch basket - the primary drive gears being of the split interference type. The two springloaded primary drive gears don't actually mesh together, they simply drive a common gear. Inboard of the front crankshaft's primary drive gear is the gear drive to the counter rotating balance shaft that lies between the two cranks. This balance shaft carries two weights, the rotation of which is designed to cancel out the imbalance in the 180° crank throws. The two cranks are arranged so that the right front piston is at TDC the same time as the left rear piston. Likewise the left front and the right rear pistons reach TDC together. As well as driving the counter-balance shaft, the froDt crank also drives the coolant pump via a gear located outboard of the primary drive gear. A conventional but solid-looking wet, multi-plate, cable-operrle clutch is employed, while the only unusual thing about the six-speed transmission is the use of pressure and not splash lubrication. This feature was also adopted a couple of years ago on the TZ250s and before that was used on the TZ750 four cylinder racers. Likewise, the cooling system is straightforward, being sealed with a remote overflow and recovery tank. To aid cooling in slow going there is a thermostatically controlled electric fan, located behind the aluminumcored radiator. The volume necessary for a twostroke's exhaust system is difficult to accommodate on any bike, but when that bike has four cylinders, all of which need an exhaust system of very finely regulated dimension, and that same bike is also destined for road use, then you've got a problem. Yamaha has chosen to route the pipes from the front cylinders back under the bike in a conventional manner while the pipes from the rear cylinders 21