CONTINUOUSWAVE

I recently bought a used 1996 Mercury 60-HP three-cylinder two-stroke-power-cycle loop-charged carburetor outboard engine. The engine idle speed is rather hight, 1,800-RPM even after warm-up. The engine also often bogs when accelerating.

I cannot reduce the engine idle speed to be lower. All the carburetors are closed and the ignition timing is retarded as far as it can go.

I suspect [the cause of the faster idle is due to] an air leak.

Give me advice on what other defect in the engine could cause the unusually high idle speed.

ASIDE: I have never before now owned a Mercury three-cylinder two-stroke-power-cycle loop-charged carburetor outboard engine.

Although not at all familiar with this particular Mercury engine, on some classic two-stroke-power-cycle engines the engine control unit is able to advance or retard the spark timing. This is separate from the mechanical throttle linkage that affects spark timing.

The faster engine idle speed that may occur during a cold start is created by an electronically control advance in spark timing. A temperature sensor switch detects then the engine temperature has risen to the normal operating level, say 130-degrees. When the switch closes (or perhaps opens, depending on the circuitry) this caused the engine control unit to revert to its normal engine spark timing. The engine idle speed is reduced by the reduction in spark timing, and returns to the mechanically set spark advance setting.

The reason that the engine idle speed persists at the fast idle even after the engine warms up could be that the thermal sensor has become disconnected, so the the engine control unit does not receive the signal that the engine is warm. Thinking the engine is still warm, the engine control unit persists with the fast idle. (This actually happened to me on an OMC engine; the connection to the thermal sensor had become dislodged. Reconnecting the sensor to its wire lead cured the problem of fast idle after warm-up. I have no ideal if this feature was included in your Mercury engine.)

The reason the engine is bogging on acceleration may be because you have adjusted the mechanical spark timing linkage to be too far retarded in your attempt to affect the engine idle speed.

Another possible cause of the engine bogging on acceleration could be the spark plug gaps have eroded. Again, I experienced this occurring with an older two-stroke-power-cycle engine, albeit another OMC V6 engine. If the engine exhibited any tendency to hesitate when asked to accelerate under load, it was a sign that the spark gap was too wide (from erosion of the electrode. Resetting the spark plug gap cured that problem. However, as I recall some Mercury engines were using gap-less or surface-gap spark plugs, so there would not be much effect from electrode erosion. But spark plugs fouled by the incomplete combustion that occurs with retarded spark timing may be affecting the spark plug operation.