Joined: Jul 24, 2007
Posts: 36855
Location: Studio_City, CA
PostPosted: 2008-07-02, 17:50:51 Reply with quote Back to top
The common wisdom is the voltage at the coils should be no more than a volt less than battery voltage. So, if your battery reads 12.5 volts, the coils voltage should be at least 11.5 volts with the ignition and run switch on. Personally, I'd rather see them getting much closer to battery voltage, and this mod will help do just that. It basically increases the current capacity to the coils so they fully charge more quickly, necessary for getting the most out of a Kettering type ignition, especially at higher revs, or when starting the motor.
Also the stock coils have always been known as weak. A popular upgrade was the green 3 ohm Dyna coils back in the 80s (model DC 1-1) and they're still damn fine replacement coils. Accel also makes 3 ohm coils that work (model 140403). The Dynas are easier to mount.
Either will work even better with this relay mod.
Using the relay isn't so much to fix a failure as much as it gets you an easy 5 to 10% stronger spark no matter what coils you use, because it provides a much more direct power feed to the coils straight from the battery.
The effect is almost like increasing the dwell time on a points ignition because the better current source allows the field to more fully develop quicker in the coils so there's a stronger field to collapse. Feeding the coils with wiring that runs all through the handlebars and stuff wasn't a great idea in the first place.
It will help with cold starting and high speed performance, where the stock ignitions are inherently weak.
It's kind of like powering your high powered horn through a relay instead of just a switch. They work much better that way, too.
I put most of the info you should need right on the drawing.
IMPORTANT! - Be sure to use a weatherproof relay!
The wire sizes specified are way oversized just so the wire is physically robust, not for current handling considerations.
Smaller wire like 16 Ga. can be used for the power feed and even smaller like 20 Ga could be used for the relay field coil feed.
I will probably just use the pigtails on the socket and fuse holder I bought for all of that on mine. Possibly use the green
"where does this wire go?" wire for the relay ground .
BTW, none of the specs that relays might have address whether or not it will latch and stay latched when a weak battery goes lower in voltage when you hit the starter button though. So I'm just saying it's something you might want to investigate before you're at Harry's Bar and Grill late at night, and it's cold outside, and the battery is low and it won't even fire, because that relay won't close.
You can just get a bunch of small batteries in series until you have about 9 volts measured with a meter, and see if that will trigger the relay to test it. If it does, you should be fine.
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