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Ok, so I am a moron, please bear with me, and don't be afraid of insulting my intelligence with basic concepts. crazy

I have an RE single, 250cc. I gave it a test run using a 12V coil from a triumph trident out back, and the 12V battery from my suzuki. It ran very well, had good power, etc. but the coil got very hot.

so I've read a few things here about using a ballast resistor to drop the 12V down to 6V, but they were mostly in the norton board and related to twins.

so what I need to know (before I burn up all my coils) is what do I really need to safely run this system with a 12V coil? can I take the ballast off a twin or triple and use it for this single?

I do also have a 6v coil on hand if needed?

please speak very slowly, as I am a chemist, not an electrician. wink thank you!


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Double check the primary resistance of the coil that's getting hot.
It should be 4 ohms give or take.(making sure it's really a 12volt coil)
If it is a 12 volt coil, the reason it's getting hot is too much dwell.
That is, the points are spending too much time closed.
That has to do with the shape of the breaker cam and the points gap.
The first thing to try is to open the breaker gap to say, .020.
Run it and check coil temp. Don't forget, that will advance the timing.

If that doesn't help, it won't hurt anything to try a ballast resistor in series with the battery feed to the coil.
Any source for the resistor, bike or car, but it should be checked for resistance which should be about 2 ohms.

The two ohm resistor in series with the 12 volt coil will mean the coil now receives 9 volts, which is plenty for spark on that bike and the coil should cool off regardless of dwell.

>>sb



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I would invest in a new Japanese 12 volt coil. A ballast resistor is something else to pack up. A Japanese epoxy encapsulated 12 volt coil will set you back $20 and last a lifetime. My 2c


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Any specific suppliers where these Japanese coils can be obtained?

beerchug


Originally Posted by andrewinpopayan
I would invest in a new Japanese 12 volt coil. A ballast resistor is something else to pack up. A Japanese epoxy encapsulated 12 volt coil will set you back $20 and last a lifetime. My 2c


Jon W.


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-> so I've read a few things here about using a ballast resistor to drop the 12V down to 6V, but they were mostly in the norton board and related to twins.<-

This is because Norton twins use 6V coils in a 12V system. (Why? I don't know.) The ballast resistor brings the voltage down to 6V. By the way, the 6V Norton coils look just like OE Tri/BSA 12V coils.

So make sure that is indeed a 12V coil - do the resistance test suggested by Sboyd. If it is, and it's getting hot, then there's probably something wrong with the coil.

(I hadn't heard about the dwell time problem before, but it makes sense, especially if the point gap is closer than spec.)


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so I assume this is a 12 volt as on the bottom it says
17M12
455221A
13 71 1

while my 6V has a 17M6. I did check resistances of five coils, 2 on my thunderbolt and these three on the trident frame, and all had relatively similar resistances. one was 5.1 Ohms, while the other two were 3.6, and both the thunderbolts were 3.6-ish, so maybe I grabbed that one.

thanks!!


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Yup, it's 12V and it should be fine. Odd about the lower resistance on the one coil, but that shouldn't make much difference. There are two ways to get the coil hot: too high an input voltage or too long of a dwell. I'd make sure the points are gapped right and it has the correct AAU in it. Also, maybe the one coil is defective?


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actually the one coil was higher in resistance than the other four tested, so that might explain it getting hotter?

as for the dwell time, with a single cylinder, the points are closed for something like 300 degrees of the rotation, no? the cam only has the one lump, so it is charging up the whole time between sparks? that might explain the longer dwell.

is there supposed to be a different coils used for a single than a twin or triple? I kind of thought a coil was a coil...?

I'll pull a different one next time and try that.

thanks!! smile





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hello snowbeard

The high resistance coil will draw less current and therefore run cooler than a low resistance coil, all else being equal.

To help you understand, a 100 watt light bulb has less resistance than a 50 watt bulb. So the 100 watt gets hotter.

As for the breaker cam
a battery and coil breaker cam for a single cylinder should be shaped as a circle with a flat side on it. This holds the points OPEN for 300 degrees, then lets them close for 60 degrees. (that's the dwell). Then when the points re-open the spark occurs.
My degree numbers are approximate and the dwell may be 40 degrees or less, which uses less battery.

If your breaker cam is egg-shaped and leaves the points closed most of the time, that usually IME is for magneto or ET use.
You can possibly still use it, try the resistor.

..sb




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Discussions about resistance can be confusing.

For a given voltage the more resistance draws less current.
For the same given voltage the less resistance will draw more current.

It's like water. The more you open the faucet the less resistance there will be to the flow of water and thus more water will flow.

So a coil with 8 ohms primary resistance will be cooler than an 2 ohm coil running at 12 volts.

I always related it to when I dropped a brand new Snap-on 5/16 Whitworth wrench across the battery terminals of the bike I was working on. The wrench reflects a resistance of about 1 ohm, almost a dead short.

Now I am here to tell you this hurts twice. Once when the sparks ruin the surface of your brand new Snap-on wrench that you paid a day's wages to acquire and then when I was stupid enough to reach down and try to remove it from the terminals with my fingers... The lower the resistance the more current will flow. The more the current flows the hotter things get.

Mr. :), As Mr. Boyd said. I do not think you have the correct auto advance cam profile for a battery ignition. The cam profile you describe, and resultant 300 odd degrees of dwell, is suited for an Energy Transfer system. WIth current flowing through teh coil for that length of time it is now wonder the coils are getting hot!
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oh my, it's a very good thing I disclosed what a moron I am up front!! laughing

I will have to look more closely at my cam then! I was under the impression that all points systems worked the same, so I really didn't inspect it with that in mind!! I was going from memory, supposition and vague recollections when I said it was closed most of the way round.

but it also used to be set up for a 6V system using only one winding of the alternator to charge the battery and power the coils, with a switched system to use the other two windings for the lighting system when required. I may have to go back to that setup if I find I was right about the cam.

of course I don't even know that the alternator works yet, I was just trying to see if it would run at all with a fully charged battery on it's own, and it certainly did. hopefully it will run as well once I have it all put right (if I find I need to change anything at all!)

it is dangerous to let a chemist muck about with these things, who ever sold him that machine!?!?! blush

thank you all very much!


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that's a good story about the wrench too, John, kinda like the old joke, "ever see a match burn twice?" wink


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Snowbeard

I see from your other thread that your 12 volt conversion is going well. I would not consider turning back to 6 volt over this coil problem.
Was the bike running on the 6 volt system in your possession without any coil or points problems?

Please take the time to do the following and post back:
Remove spark plug and rotate the engine while observing points.
Determine how many degrees of camshaft rotation the points are closed. Points closed can be determined with paper, but is best with a meter.

Now, set the points gap to .020 at widest point on cam.
Repeat test.
>>
Steve


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Hi Steve,
thank you very much, I really appreciate the help. I will go out and set that gap as soon as I can.

So when I got the bike it had been stripped of most of it's wiring. the wires from the alternator come into the airbox, three of them, one green black, one green yellow, and the third white green. they all go into a three position Lucas looking junction, and have the proper looking bullet connectors. I wonder if this may have been replaced with one from a triumph based on the fact that it matches the colors of Derry's cub alternator?

the contact wire also comes into the airbox. that is it. the headlight has been replaced with what is probably an indian made new one, 6v. it is super shiny with two wires, red and blue (one for high and one for low, assuming the housing is ground) and it has the Lucas taillight, with short wires and bullet connectors.

otherwise I'm starting from scratch. I was going to purchase the original Lucas seven point switch from Hitchcock's but then heard that the alt should support the 12V conversion, so I figured I'd try that. once I made that decision I just wanted to hear if the fellow was right when he said the engine was solid, and all it needs is wiring and cosmetics.

as you saw in the RM-13 thread, it seems that things are in fairly good order! I am very happy to be this close! thanks again! :-)


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The way it was explained to me, many years ago, is that there is no such thing as a 12V coil. Coils naturally want something between 3 and 6 volts (nominal). So any coil being run on more than 6V need a bit of resistance to lower the voltage. Sometimes this is built right into the coil and sometimes it is added externally, usually in the form of a ceramic ballast resistor or a resistance wire built into the wiring harness. However, the coil designed to be used with external resistance may still be labeled as a "12 V coil". So what you got to do is figure out what kind of coil you have, then add resistance as necessary. The homemade points system on my bike uses a big coil made for GM cars, and, for 12V, need a 3.3 ohm ballast resistor. Without that, it gets real hot, an the points burn. You can get a ballast resistor at any auto parts store.
Note that if you are converting a 6V bike to 12V that you don't need to replace the coil - you can just add a resistor.


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