Coil Breaking Down?

[JAG59]

I have a 1977 Merc 40 4hp Serial # 91491451. I thought that I had a carb problem. Darn thing wouldn't get out of it's own way. It would bog down if given more than 1/2 throtle. After cleaning the carb and getting no change in performance, I found that one of the plug wires had pulled out of the socket but still gave the appearance of a good connection because it was hidden in the wire boot. Now it acts like the coil is breaking down and not firing correctly. My question: If this engine was used for an extended period of time with the plug wire disconnected could it have overloaded that coil and caused it to fail?

 

[ricksrster]

Re: Coil Breaking Down?

You can check the resistance of the coil windings. The primary, where the green wires connect to where the black wires connect should be 0.2 - 1.0 Ohms. The secondary, from inside where the plug wire connects shoild be 800 - 1100 Ohms.

Your problem also could be timing and you need to do a link and sync.

Also it could be the fuel pump or primer bulb.

 

[JUSTINTIME]

Re: Coil Breaking Down?

to check primary, just unplug the wires to the coil and test in ohms, u want less than 1 ohm
seconday, go from coil ground to end of spark plug wire u should have a at least 1K

 

[Texasmark]

Re: Coil Breaking Down?

Yes.

What happens in the coil is that when the voltage has risen to the designed value it jumps the plug's gap and releases the energy stored in the coil. That puts a clamp on the max voltage available internally in the coil.

If there is no gap breakdown, the coil could theoretically go to infinity, but due to design of the things and losses it dissipates before it gets there. It finds somewhere to dissipate and therein is your damage.

If it only does it once or twice so to speak, it won't produce a carbon track. But repeatedly, it burns up whatever it's jumping across, making carbon and the longer it runs the shorter the carbon trail making the malfunction that much more intense.

Mark

 

[JAG59]

Re: Coil Breaking Down?

I just want to reply to this one last time and put this thread to bed. I replaced the coil and the engine now works flawlessly. I did find a tell-tale carbon track on the old coil where it was arcing through the plastic case to the mounting bracket. Thank you to Texasmark and Justintime for there help. Texasmark, your explanation really helped me to visualize the situation and to even explain it to some friends and fishing buddies. Thanks again.

 

[Texasmark]

Re: Coil Breaking Down?

Glad you got her solved.

Mark

 

[JUSTINTIME]

Re: Coil Breaking Down?

good to hear