Re: mercury spark advance
Thats the idle speed control. It works to maintain an even idle speed on the low end and it pulls timing on the top end to give sort of a soft stop rev control before the ECU rev limiter kicks in. The idle speed control boxes have a reputation for going haywire and advancing timing over factory specs and ruining motors although to be honest I don't even think that is physically possible for the timing controller to advance timing further than its set point. Retard timing sure but to advance timing the timing box circuitry would have to be able to anticipate a timing signal. I just dont see it happening. In any case many people, including myself, disconnect them and time the motor for WOT, set the idle speed and call it a day. You will pick up speed on the top end by doing so.
This is where all tha "rumors" comes from. Its in tha service manuals.............Like you, I've never really seen it happen.
TROUBLESHOOTING IDLE STABILIZER SHIFT SYSTEM
When outboard is idling IN NEUTRAL, shift switch circuit is in the OPEN
position and system is INACTIVE.
When outboard is shifted INTO GEAR, shift switch circuit CLOSES. BIAS
VOLTAGE from each switch box is changed by a 6.8K (?.34K) resistor located
in the WHITE/BLACK lead between the switch boxes and the shift switch. The
shift switch is now CLOSED and completes the circuit to ground. THREE
DEGREES of timing advance occurs when the shift system works properly.
If the resistor is OPEN or the shift switch circuit stays OPEN, the THREE
DEGREES of advance will not occur when the outboard is shifted into gear
AND maximum timing at W.O.T. will be RETARDED THREE DEGREES.
If the resistor should SHORT TO GROUND, engine timing will be overly
advanced and damaging powerhead detonation will occur.
