As the maximum power range of operation (75% to 100% load) is approached, some device must allow richer mixture (A/F about 13: 1, F/A 0.08) to be supplied despite the compensating jet. Such a device is the meter rod economiser shown in Fig. 10.31. The name economiser is rather misleading. It stems from the fact that such a device provides a rich uneconomical mixture at high load demand without interfering with economical operation in the normal power range. The meter rod economiser shown in Fig. 10.31, simply provides a large orifice opening to the main jet as the throttle is opened beyond a certain point.
The rod may be tapered or stepped. Other examples provide for the opening of auxiliary jets through some linkage to the throttle movement or through a spring action when manifold vacuum is lost as the throttle is opened.
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