The inlet metered DB2 pump (Figure 4-60) has a pumping period with a variable beginning and a constant ending. That is, at minimum throttle positions the metering valve is only open a small amount, so the plungers only move a small amount. And the rollers have to ride a great distance up the cam ramp before they can cause the plungers to pressurize the fuel. This causes retarded injection timing as compared to wide open throttle position (maximum metering valve opening). Because at WOT; plunger pressurization begins as the rollers start up the cam ramp, and peaks at the lobe. Therefore, timing must be advanced at light loads (part throttle) to compensate for this injection timing lag. The amount of light load retard is compensated by an inherent light load advance obtained from the mechanical light load advance.
