Prior to the 1980s, Husqvarna had designed only two-stroke motorcycles for decades. So, when they designed a new 510cc four-stroke motor, they based it on their existing two-stroke bottom end and employed quixotic but logical engineering to lubricate the engine. Rather than a positive-displacement oil pump and dedicated oil lines, the engine utilized nothing but changes in internal engine air pressures and a pair of tiny, simple reed valves to coax the oil where it needed to go.
As the piston rose, it created a vacuum the crankcase, which sucked oil through a carefully designed set of orifices to lubricate major engine components. When the piston came down, one reed valve would shut and another would open. The resulting pressure would push oil up through the cam case to lubricate the cam chain, overhead cam, and valves. The other components in the engine and transmission were splash-lubricated by the spinning of the internal components, just as they were in a two-stroke engine. The engine housed a scant pint of motor oil.
The result was a remarkably light, compact, and simple engine that didn’t look all that different from their two-stroke models. Surprisingly, the super basic oiling system was quite robust, and the engines were fairly bulletproof. The odd lubrication technology’s only drawback was inadequate oiling at steady throttle settings, making the system inappropriate for a freeway flyer.