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Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 11079
Experience:  40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
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1985 Yamaha 40 hp (40ELK) - been running real rough at idle

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1985 Yamaha 40 hp (40ELK) - been running real rough at idle for some time. Now no Spark to any of the three plugs. Engine kill switch lanyard in place.

Hello. My name is XXXXX XXXXX I will be assisting you today.


Disconnect the white wire from the key switch and kill switch now check for spark. If spark returns the key switch or the kill switch are shorted to ground. Reconnect each switch and check for spark.


Post back with findings.


Do you have a ohm meter to do some testing of the ignition components?

Customer: replied 3 years ago.

I will try these checks - Yes I do have an Ohm meter

Hi Doug, If you still have no spark unplug the wiring harness from the engine and check for spark. To crank the engine locate the start solenoid and unplug the small brown wire and apply 12 volts to the small brown wire of the solenoid. This will allow you to crank the engine.


Post back with findings.

Customer: replied 3 years ago.

unplugged the white wire at kill and key switch - still no spark will try unplugging wire harness now.

If still no spark with the ohm meter test between the blue and brown wires of the stator should have 180-250 ohms.
Customer: replied 3 years ago.

214 ohms

Customer: replied 3 years ago.

Did I loose you?

That leaves the CDI unit # XXXXX as bad. click here


There is one more component we can test the trigger but usually all 3 coils don't go out at the same time.


Test from black to the white/red, blk to white/green and blk to white/black 310-390.

Customer: replied 3 years ago.

Please explain the last test - Trigger?

The trigger tells the CDI unit when to fire the ignition coil. There are 3 sensor coils one for each cylinder. The CDI unit is the most common failure. The stator produces the voltage that powers the CDI unit.
Customer: replied 3 years ago.

Is it possble a bad CDI unit would make for a rough idle?

Yes as it could be misfiring due to weak spark or just a misfire of one cylinder due to loss of spark.
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