Magneto impulse coupling

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Erik

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Oct 10, 2017
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Several years ago I built an Fairbanks Morse fmx2 magneto (nos army surplus @15€ :)) on 850 mkii cases.Old alnico magnets so no temp riscs. Runs great, but can be hard to start. Just found a lucas sr1 magneto with impulse coupling. Hefty spark at very slow turn! Anyone tried such a coupling on a Norton?

Erik
 
Triton Thrasher said:
Is there room for it?

I must say, kickstarting rpm has always been enough to make any of my magnetos start the bike.
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I think its possible, but does need quite a bit of engineering. On the other hand, it might also be the choke cicuit on de keihin crs carbs i use. It either starts first kick or takes 5-10 tries.
 
Matchless said:
Impulse coupling?
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Sounds odd but it seems to work on planes, but how the timing is maintained if the magneto is spun faster than the engine is suspect.

A magneto with an impulse coupling is a device used in airplane engines that provides a boost to the magneto's rotation to generate a strong spark for starting the engine:


  • How it works
    The impulse coupling is a mechanism that sits between the magneto's drive shaft and the engine's accessory drive. When the engine starts, the impulse coupling winds up a spring, then releases it to give the magneto a boost.
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kommando said:
Sounds odd but it seems to work on planes, but how the timing is maintained if the magneto is spun faster than the engine is suspect.
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Well, that's a new one on me, & as you say, how is the timing maintained?
I think a Lucas SR type magneto would work fine without any assistance. The K2Fs on my Atlas & 650ss work just fine & the bikes are easy to start, unlike my Big Head Bullet with the Electrex World ignition!
 
kommando said:
Sounds odd but it seems to work on planes, but how the timing is maintained if the magneto is spun faster than the engine is suspect.
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I’ve only seen them on old stationary engines.

Timing when you’re hand cranking an ancient Lister can be approximate and still work well enough to get it started. Once it starts, the impulse mechanism is thrown out of engagement by a centrifugal trigger.

The hand cranking is probably very low rpm: lower than kickstarting a bike.
 
Modern magneto’s like champion slick (for aviation engines) also use impulse coupling. The coupling is spring loaded, under normal operation the spring pushes the rotor against a stop on the drive shaft. If the magneto rotor is blocked by a pawl, the spring winds up further, but only for a mechanically controlled angle (lag or retard) when it reaches this angle is automatically releases the stop pawl. Rotation speeds up and, breaker opens and magneto sparks like at high speed. When de engine reaches operational rpm the pawls are blocked centrifugally.

I have got a sr1 with impulse coupling. It gives an amazing spark at extremely low rpm, but feels a bit like a mousetrap. I reckon if its ok for a plane it must be ok for a motorbike.
 
This may help with understanding the why’s, do’s and do not’s regarding impulse couplings . As far as impulse couplings increasing complexity and consequently reducing reliability it is my understanding that aircraft engines have two mags for redundancy but only one will have an impulse coupling .
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Magneto impulse coupling
 
Ohyeah, like that shower of sparks, but one magneto is just all I can handle on the Norton and my wish to loose the battery was what it all got started… ;-)
 
Joe Hunt magnetos will fire up on slow spark or a lazy kick no need for IP coupling even at full advance.
 
Erik said:
Ohyeah, like that shower of sparks, but one magneto is just all I can handle on the Norton and my wish to loose the battery was what it all got started… ;-)
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When I first installed my Joe Hunt magneto I ran the bike without a battery and ran my lights from a battery eliminator, worked great but when running the head light and if you hit the brake the head light would go dull, I didn't ride the Norton at night much but sometimes getting home late when the sun goes down, but was all good for day light riding and having a bright brake light, but these days I run my small dirt bike battery from my Honda just for brighter brake light, horn, I don't run daylight light at all, bikes before 1985 here no need to run daylight light, but if I get caught out in the dark I will have bright head light and tail light, the Honda battery is small and not much weight.
The battery sits under my seat in the tool tray with all my electric connections, it might look like a mess but works great and haven't had any problems in the 15 years its been there, the battery eliminator/rectifier is mounted at the front of the tool tray where the ALT wires connect to and to my wirering system, I can still run the light if the battery shit itself as well the JH maggie needs nothing to run my bike.
I run a very simple wirering system as you can see beside the eliminator is my main wire harness consisting 4 wires 2 main power wires going to the head light and 2 wires for the front brake light, and the red wire is my earth wire I don't run my earth wire inside my main harness.

Magneto impulse coupling


Magneto impulse coupling


Ashley
 
Daylight lighting is a good isea around here. Idling around 1200 rpm gives no rpoblems with lights. If there are, you can alway resort to LED. Don’t like batteries, esspecially in old motorbikes or cars, short circuit waiting to happen.
 
Well, rebuilding the sr1 to sr2 is not so good I think, the magnetic rotor is not symmetric. Hmm, so keep it sr1 and go to enginespeed? Still need distributer then, and lots of engineering at the distribution side. And probably risc of point bounce on hig rpm. impulse is also difficult. Turn out it is partly acitivated by gravity. Since the magneto is not upright I need one with a adjustable dog angle and counter clock wise rotation. Too difficult as well.

So if I want a a single fire solution and preferably Lucas it needs to be an original SR2, or a single fire Fairbanks, but well, that’s not from the motherland, right? ;-)
 
Found it! SR2 in good shape. Traded with Tim, he needed a SR1, so restored one for him. Check out his site, search for surrogate manx. He is building a racer on a budget 2500€…
 
Did some measurements, the dc resistance of the coil of the fairbanks magneto is 12kOhm, the Lucas coil resistance is a little over 6 kOhm. I think logical, the fairbanks is dual fire so needs more Volts for the wasted spark. But it also means that the current from the lucas could double the fairbanks if all else is equal. It’s the current that give’s the spark it’s power. Hmm, promising?
 
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