Why This Document
I am often asked about certain components that make up the ignition system, specifically about the spark advance module and the idle stabilizer module. I will try to explain what each component does, and also explain why you should, or should not have these installed on your motor. Keep in mind! This is just my opinion based on knowledge gained from listening to people that know far more about these motors than I ever will. Before understanding what these components do, I will try to briefly explain a little bit about how the ignition system works.
Basic Ignition System
Stator
This part serves two purposes. It provides voltage to the regulators for keeping the cranking battery charged. It also supplies the primary voltage to the switchboxes that eventually goes to the coils and provide the spark voltage for the spark plugs. In essence, this is where the fire begins. It is located at the top of the motor underneath the flywheel.
Trigger
The trigger is what tells the switchboxes to "fire" and send stored voltage from the stator to the appropriate coil which eventually provides the "spark" at the spark plug. It is located at the top of the motor under the flywheel in the center of the stator.
Switchbox
There are two switchboxs that do a simple task, but in kind of a complicated way. They basically take a primary voltage from the stator, and store it until the trigger tells the switchbox to send that voltage to the coils.
There is also a circuit within the switchboxes called the Bias Circuit, which basically manipulates the signal from the trigger, and can either advance or retard the ignition timing depending on conditions.
The switchboxes are located on the starboard side of the motor, and are roughly a little larger than a deck of cards. They are stacked on top of one another with several wires connected to them.
Coils
The coils take the voltage that was stored in the switchboxes and increases it enough to jump the gap at the spark plug, thus providing the "spark". There are six of them (one for each cylinder) located on the back of the motor.
Problem Components
This brings us to two components installed on older 2.5 liter motors that in my opinion, should be removed. The first is called a Low Speed/High Speed Spark Advance Module, and the second is the Idle Stabilizer Shift System.
Low Speed/High Speed Spark Advance Module
This module does a few things that can be quite dangerous to your outboard should it fail. It is typically located either on the top aft part of the motor, or on the port side of the motor. It is about the size of a pack of cigarettes, and has a white wire with a black strip, a red wire with a white stripe and a black wire coming out of it.
What this does is manipulate the bias voltage on the switchboxes under certain conditions. First off, if engine idle speed falls below 550 RPM, it will advance ignition timing by as much as 9 degrees. It also will advance ignition timing by 6 degrees when the engine reaches 5000 RPM, and retards ignition timing by 4 - 6 degrees when the engine reaches 5600 RPM.
The danger is the manipulation of engine timing, specifically advancing the timing via the bias voltage. Should it over advance the timing or advance it at the wrong time, engine detonation and failure will occur. On the "flip-side", retarding engine timing robs your motor of power, and can cause poor operating peformance.
Idle Stabilizer Shift System
The second component that I would remove is called the Idle Stabilizer Shift System. This consists of a 6.8k resistor that looks like a 1 inch long cube. It has a white wire with a black stripe that connects to the bias circuit of the switchboxes and a switch tied to the shift linkage.
What this does is advance ignition timing 3 degrees when the outboard is shifted into gear to keep the motor from stalling, especially if using a higher pitched prop. The danger of this is should the resistor fail, or the wire short to ground, engine timing will be over advanced and once again, detonation and power head failure will occur.