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Ironhead Sportster Motorcycle Talk (1957-1985) For all those that wanna talk about Ironhead Sportster Motorcycles

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  #41  
Old 8th August 2020
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from 2 weeks ago.http://xlforum.net/forums/showthread...33#post5838433
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  #42  
Old 8th August 2020
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On our poor old slow revving Ironheads the stock battery/coil ignition is still capable at 7000 rpm. But it's marginal.
The problem here is what we call 'dwell time" and it's the time the points are closed. While the points are closed current flows through the coil's primary circuit until it reaches "saturation" (think of a sponge).
But as an engine spins faster there is less and less time for the coil to reach this point. In fact, it might not get there. At that point the available energy is less than it should be so when the points open and the magnetic field collapses it will produce less voltage at the plug. Hence, a weaker spark. Possibly not enough to light the fire and you now have a "miss".

It sounds just like you have hot the rev limiter which is in fact, what you have just done.

In the "olden days" racers used to set the points gap by "dwell time". That is, they would reduce the points gap until they had sufficient dwell to saturate the coil and keep the engine running at higher rpm's. I still have a dwell meter in my toolbox.

They developed dual point distributors to help overcome this problem but of course the real answer was electronic ignition with it's much faster switching times or in the case of serious race engines, magneto's.
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  #43  
Old 8th August 2020
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I feel a bit sorry for the younger guys who now have to spend many minutes on a laptop, tweaking their advance curve, agonizing over where to set the rev limiter, reducing or increasing the dwell time.

I'm not sure why this can't be fully automated from the O2 sensor and tach feed.

Oh yes. I suppose they do have them.

I have seen what they can do with drones. How long before Moto GP bikes are riderless with the controllers sitting in the pits ?

Wait ! Couldn't robots replace the controllers ?

Then we could all go to the pub and watch the races on the big screen.
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  #44  
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Interesting.
Thx FE
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  #45  
Old 8th August 2020
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Quote:
Originally Posted by Ferrous Head View Post
On our poor old slow revving Ironheads the stock battery/coil ignition is still capable at 7000 rpm. But it's marginal.
The problem here is what we call 'dwell time" and it's the time the points are closed. While the points are closed current flows through the coil's primary circuit until it reaches "saturation" (think of a sponge).
But as an engine spins faster there is less and less time for the coil to reach this point. In fact, it might not get there. At that point the available energy is less than it should be so when the points open and the magnetic field collapses it will produce less voltage at the plug. Hence, a weaker spark. Possibly not enough to light the fire and you now have a "miss".

A standard Harley points igniton triggers the coil 2 times for every revolution of the point cam. If the Harley has coil saturation problems then I wonder how a 1960's era solid lifter V8 could rev to 6000 - 6500 rpm while triggering just one coil eight times with every revolution of the distributor.

The dwell angle for the V8 is about 30° and about 140° for my '74 sportster. A big difference in time.

Just thinking here.
I can't say for sure that lack of coil saturation isn't a problem but I see no evidence of it on my stock bike. It's mostly the stock exhaust that hurts it at high revs.

Another question. Does the Harley coil need more time to saturate because it is firing 2 plugs at once? I don't know. Over my head.
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  #46  
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Originally Posted by Ferrous Head View Post
I still have a dwell meter in my toolbox.
me too.it's part of my engine analyzer last used on the points of my'74 pontiac grand am.the distributor had a sliding window that opened for my t handle allen wrench to set the dwell.there was a range for point gap iirc i used to set it on the narrow end of the spec to increase coil saturation time.
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  #47  
Old 9th August 2020
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Quote:
Originally Posted by needspeed View Post
A standard Harley points igniton triggers the coil 2 times for every revolution of the point cam. If the Harley has coil saturation problems then I wonder how a 1960's era solid lifter V8 could rev to 6000 - 6500 rpm while triggering just one coil eight times with every revolution of the distributor.

The dwell angle for the V8 is about 30° and about 140° for my '74 sportster. A big difference in time.

Just thinking here.
I can't say for sure that lack of coil saturation isn't a problem but I see no evidence of it on my stock bike. It's mostly the stock exhaust that hurts it at high revs.

Another question. Does the Harley coil need more time to saturate because it is firing 2 plugs at once? I don't know. Over my head.


I suspect the rise time for the coils depends more on construction of the coil. The number of turns in a coils winding probably has something to do with it.

More expensive coils would most likely be those having more turns (higher voltage levels) to produce hotter sparks. I'm no electrical engineer I am only going o what I've read and my own experience.
But no, never believed a coil was not fully saturating on a Harley UNLESS it was failing.
I have never bothered even checking the dwell time on my HD's. Never believed it to be necessary.

HD probably should have paid Nicolas Tesla to design their coils.
Wouldn't have needed batteries then !
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