Bore Stroke: 85mm x 87mm
Swept Volume: 493cc
Max Power Output: 41kw
Max RPM: 8600rpm
Compression ratios: 13-14
Carb Diameter 34mm
Spark Plugs: PAL Z15
Ignition Advance: 36 deg
Valve Clearances (Cold):Inlet:0.15mm; Exhaust:0.20mm
Oil Castrol R SAE 40
Transmission Ratios 9.95 - 11
Front Wheel 2.75 - 23"
Machine Weight: 116kg
Vertical single cylinder OHC with swept volume 500ccm. Engine oil sumps, head and cylinder are made from aluminium alloy, cover-lid of final drive and ignition are made from that of magnesium. The working position of cylinder has "Nikasil" modification. The big end and main bearings are of needle and cage type.
Camshaft is seated inside the head on needle bearings and it is driven with a chain. A forged piston is made from light alloy with two piston rings. Oil Tank is situated in the cover of valve gear and the R.H of crank case. Its level can be checked after unscrewing the screw of the final drive cover, which is located at the right half of the final drive cover over the axis of crank mechanism.
The frame is welded from steel tubes, its middle and back sections are screwed together. Front fork is telescopic. Rear wheel is unsprung. Steering column is embedded in casings. Fuel tank is seated elastically.
Clutch plates are taken along with internal teeth on hub, thus resulting in good operation. A sufficient clearance should be checked between the thrust plate and clutch hub at complete switching (1mm at minimum).
The clutch is of multi-plate type. Its proper operation depends on an accurate disengaging of the clutch. When the clutch is disengaged, the thrust plate must move away from the plates uniformly, this can be achieved with the uniform tightening of nuts of springs at the clutch cover lid. A proper operation of the clutch is checked when the clutch is disengaged and the rotating with rear wheel must be possible without resistance.
Adjustment of carburetter
Motorcycle is provided with the Italian make carburetter "Dellorto" of diam. 34mm with choke, corresponding to the FIM regulations.
Main adjusting values:
Main jet 205 - 220
Needle jet 290 - 300
Sleeve valve no. 8
Needle position - the second cut from the top
Way of adjustment is as follows:
A part of the track is driven at full-throttle. After a rapid choking and stopping of the engine, (it may not run idle) the fitness of main jet is controlled. If the jet has been chosen properly, the piston bottom is lightly moistened with traces of oil and gilded. If the needle is small (lean mixture) the piston will be dry and greyish. These symptoms are evident at the plug too. If the jet is great (rich mixture) there are patches of fuel at piston and plug. To determine a right jet which represents the most important part of the carburetter set-up, the carburetter transition is adjusted by means of the idle-run screw and sleeve valve needle, provided with different cuttings. The highest engine power output can be achieved only if the carburetter is adjusted in this way. To prevent the piston from burning-up the jet, being greater by single grade, is recommended to be chosen while the engine power output will decrease only slightly.
All the crews of frame and wire spokes should be tightened at first runs of a new motorcycle. Steering head and telescopic fork will be lubricated with grease. Chains should be kept properly tightened. After each race, the primary chain should be washed out and lubricated inside a warm bath of oil and graphite. The accelerator handle should be maintained in such a way that after releasing of the handle, the carburettor sleeve valve will be completely closed up.
After race season, the fuel tank must be discharged and cleaned, together with cocks and carburettor, to prevent methanol from hydration and aluminium parts from corrosion. These parts are recommended to be searched through or cleaned at the beginning of a new season.
John Blackburn sent me a picture of the rear suspension of the prototype 893 machine that featured a central springing of the rear swinging fork.