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Yamaha XS500 Gallery

Although looking like the smaller XS400 and XS250 bikes, the XS500 has a different twin-cylinder engine which features twin-overhead camshafts and four-valves per cylinder. Although such an engine would be ideally suited to a sports motor cycle, Yamaha instead have used it in a state of tune for a bike they classify as a tourer with enough engine power to enable it to keep up with larger capacity machines.

The oversquare power unit displaces 498cc and with a compression ratio of 9.6:1 produces 49bhp at 85oorpm, just one brake horsepower down on the company's 650cc twin.

A four-valve-per-cylinder engine is used because four small valves use space more efficiently than two larger valves, so that, in fact, more head area can be utilised which in turn makes the engine more efficient. Also the lower reciprocating mass of small valves, means higher engine speed before 'bounce' is induced, and better ignition is helped by the sparking plug being central in the head, its most efficient position. The problem with the layout is that to get the valves in an efficient semi-hemispherical head, twin overhead camshafts have to be employed or a complicated system of pushrods and rockers which would negate the effect of the extra engine speed gained from the layout. The XS uses direct-operating overhead cams which make servicing more complicated and time consuming.

The Yamaha has pistons set at 1800 unlike many four-strokes with their 3600 layout with the inherent vibration difficulties. However, Yamaha had the problem of the crankshaft rocking with the alternate reciprocation, so to encounter this they use their patented 'omniphase' balance shaft.

The top speed of this 4251b bike is 105mph, while it will accelerate over a quarter mile from standing start in 14.3secs; fuel consumption is 46mpg. In all, the XS500 is deceptive for it is more advanced than it looks and 'four-stroke twin' specification suggest.

Bike Image Description
1976 Yamaha XS500C Yamaha XS500C Air cooled, parallel twin cylinder, DOHC, 4 valves per cylinder.
1976 Yamaha XS500 1976 Yamaha XS500 Highl;y modified cafe racer, street legal.
1977 Yamaha XS500C 1977 Yamaha XS500C  
1978 Yamaha XS500C Yamaha XS500  
1978 Yamaha XS500C 1978 Yamaha XS500C  
1978 Yamaha XS500 1978 Yamaha XS500  

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