Atmospheric Inlet Valve
Some early engines, including those of Harley-Davidson, relied on piston suction to open the inlet valve when, as the piston descended, the valve opened, with a light spring to close it again. It was a simple system, but limited engine speed to around 500rpm (that's 500rpm, not 5,000) which was fine for the pioneer single-cylinder machines. But as the demand for more power grew, so such lowly speeds became inadequate. This was made clear to William Harley in 1909, when his prototype V-twin produced little more power than the single from which it had derived. The answer, which appeared on the first production Harley twin in 1911 was a pushrod-operated inlet valve whose positive operation allowed higher engine speeds and thus more power.