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Old 02-08-2008, 15:01   #1
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Location: Houston, TX
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Coil overheating

Hello all,

I have a problem with my atomic 4 not being able to run at idle. I have to give it 3/4 throttle to get it going when i put it back to idle it dies. I've checked for spark on all the plugs and thats fine. Also I have a problem with the coil and electronic ignition overheating. Smoke was actually coming out of the coil! Ive checked the ground connection and that appears to be fine. I assume that these two problems are related but i'm not sure. Electrical is not my forte. Has anybody seen these problems before?
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Old 02-08-2008, 19:07   #2
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The most likely culprit... You are using a coil that requires a ballast resistor to drop the line voltage from 12.6v down to something more manageable. Most electronic ignitions (and point ignitions...) require a 3ohm coil. This drops the + terminals voltage.

Ohm between the + and - terminals and you ought to have around 3ohms... if it has an internal resistor. (with the key off...) 0 ohms, and you've got a shorted coil.

If your coil shows less than 3ohms, but doesn't show continuity... you can install a 2ohm ballast resistor in the positive line. Points ignitions (Kettering) use low voltage to keep from frying the points. The higher the voltage the better it arcs, and the more amp potential... great for lighting off the spark plug, not so great for stretching time between point replacement. Ballast resistors were Stock on old fords and chryslers for many years, go to the parts store and ask for an "ignition resistor." It'll be a ceramic rectangle with a coil inside, one screw mounts them to a heat sink. On cars, that'd be the firewall... on the boat, a big piece of aluminum will work. These things get really hot, so wait until someone that knows boats writes back. I don't know if they are coastie approved.

Now turn the key on, and check the voltage between the coil terminal and the positive pole. You will have to turn the motor over until the points are closed... If this is 12v, you have a dead short and no resistor in the coil. Less than 12, and you've a semi-functional coil... It ought to drop less than 9v if you have an internal resistor. With your external resistor, it'll be a lot less. (5v +/- going into the coil... No big deal, points ignitions work just fine on 6v systems too... and their coils have resistance too!)

There is a screwball here, if the switch provides 12v to the coil bypassing an external ballast resistor for easy starting, and somebody in the past wired it backwards... You can check it while its running, but... Man that fan belts mighty close!

It's also possible to wire in the electronic ignition backwards, they don't much care for that. Most cannot take 12v, and do not like having the key left on with the points closed (Dead short... Coils will let the smoke out if you leave the key on for a few hours too!)

Coils will still start the engine if you wire them in backwards as well, double check this... Coils wired backwards get hot.

Extreme point gap will increase the operating temperature of the coil as well, think charging time... Dwell still happens with an electronic box, so check it... but its not easily adjustable.

All this being said, if you have an infrared pyrometer... or quick hands, reach back there and see if its blistering hot or if you've got a little oil leak dribbling on it. Your coil is filled with oil, so if you have smoke coming out I'd bet a dollar the case has a crack for it to escape.

It's one of the grand things about electronics... when they start getting hot, their resistance goes up. When the resistance goes up, they get hotter... until they let out the magic smoke.

Hope that helped!
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Old 02-08-2008, 19:16   #3
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What he said.

You can get a coil with an internal resitor at any auto parts store.

It will be labeled as such.

I have had a number of customers that have fried coils by leaving the ignition on.

As far as wiring it will usually fry the electronics and nothing will happen.

Now as far as idle problem. I have seen this with a lot of A-4s that don't get much use. The idle jet gets gummed up and the engine will run at higher speed fine but has to be run w/choke at slower speed. Gas additives or spraying carb cleaner won't do it. The carbs are fairly simple to take apart and clean....If you need advice on that step get back to us.
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