‘75 or ‘76 Mercury 650 three banger spark issue

[csquared76]

Hello folks, first time posting here. I just replaced the OEM switch box and coil on my old Merc with the CDI Electronics unit. The boat ran before the swap, but once it got good and warm, it would stop running and wouldn’t start again until ice cold. I diagnosed no spark when hot-which led me to the aforementioned retrofit. Now however, the new coil jumps spark directly to one of the bolts holding the new CDI unit on the engine! I’ve checked grounds etc. Any ideas? I held a spark plug boot over the. Bolt head and got spark to the cylinders, but as soon as I removed the boot, it returned to arcing at the mounting bolt right beside the new coil.

 

[Chris1956]

Check the resistance of the spark plug wires. It should be zero.

Distributor ign right? The spark plug wires should screw into the Dist. cap.

Check the condition of the rotor and cap. They last a long time, but not forever. The rotor is not detachable, expensive and fragile.

 

[csquared76]

Check the resistance of the spark plug wires. It should be zero.

Distributor ign right? The spark plug wires should screw into the Dist. cap.

Check the condition of the rotor and cap. They last a long time, but not forever. The rotor is not detachable, expensive and fragile.

Thanks Chris, I was poking around online for the distributor parts…not easy to find anymore, and worth a mint. But testing those old wires would be a logical next step in this saga. I’m wondering why it ran so well before the retrofit tho…would the new coil be so much more powerful that it is more sensitive to a high impedance in the plug wires or distributor system?

 

[Chris1956]

I do not know the answer to your question. Since the coil is arcing to ground, which should be a higher resistance than thru the distributor and wires, I thought it logical to try. One side of the coil is grounded. Is that ground clean and tight?

Any chance the distributor is out of time? If the rotor was not near the cap contact, when the coil fired, it would be inclined to arc to ground, however, I am not even sure if that is possible based upon the design of the distributor.

New spark plug wires are available at the following link and probably some others. They have a stranded stainless steel conductor

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