Reference - See Page 164-53 of the Johnson Service Manual - construction and function of the mercury switch. It will be recalled that the mercury switch was formerly installed on the electric starting models only - RDE & RJE. You have noted by now that its installation has been included in the assembly of all RD'S - 19 (35 H.P. for 1957) and that the automatic cut-out switch previously grounded to the motor "direct" is now insulated by means of a hard rubber plate - Further, a ground lead has been added - extending from the mercury switch bracket directly to the motor.
Purpose of grounding the automatic cut-out thru the mercury switch is to avoid possibility of "missing" when operating at the "low end" of the fuel saver range. Operating as such, with full spark advance and partially closed carburetor shutter, suction in the manifold frequently is sufficient to cause "flexing" of the diaphragm in the cut-out to establish "ground" contact and resultant grounding of the breaker points (for lower cylinder only). However, since the ground circuit now includes the mercury (drop) switch, the circuit is open during normal operation of the motor thus, any flexing of the diaphragm occuring at this time is not effective - "missing" is avoided.
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