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Another John T Quiz Question

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John T

06-05-2000 20:18:49




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Its been a while since I put up some quiz questions, so here’s another one for fun and education which I will post the answer for shortly: Impulse Shimpulse ! Whats the deal with those mags and all that trip and spring mechanism having to tighten a spring then trip all of a sudden over and over ?? Gee whiz, when the tractor is running at high rpm isn’t that tough on the springs etc. all that tripping and why do they have to wind and trip over anyway, what’s going on?? This may seem elementary to you old timers but hopefully some newbies can learn a thing or two here. Ol John T Engineer in Indiana.

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John T's Answer

06-06-2000 16:51:13




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 Re: Another John T Quiz Question in reply to John T, 06-05-2000 20:18:49  
Well as I figured everyone hit this one right on the head. The magneto works by passing a permanent magnet past a coil of wire (no batteries required), thereby inducing a voltage between the coils ends. However, when you pass it by very slowly, it hardly produces any volatge, but the faster you pass it by, the higher the voltage produced. Therefore, at hand cranking speeds, the spring is wound and then trips so the magent passes by the coil very quickly producing sufficient voltage to operate. But at approximately 250 RPM, cenrtifugal force makes the trip dogs inoperative and you get direct 1 on 1 mag continuous turning and no more spring trips are necessary. Additionally, to retared timing at hand crank speeds to avoid flywheel kick back, when the trip dogs and impulse springs operate, the timing is more retarted then when the trips are no longer working at higher RPM where spark advance is needed. Ol John T Engineer in Indiana

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buckeye al

06-06-2000 14:48:20




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 Re: Another John T Quiz Question in reply to John T, 06-05-2000 20:18:49  
The impulse spring wind up and subsequent trip allowing the shaft to "catch up" with the engine is to increase the rate of change of the magnetic field in the coil's pole piece. The number of degrees of windup is the lag angle and is built in to the impulse coupling's design. The lag angle is equal to the number of degrees the spark is advanced between crank speed and run speed. In many mags when the engine's speed causes centrifugal force on the dog's counter weight to exceeds the force of gravity, the dogs retract and the lag angle is eliminated allowing the engine to now run on an advanced spark. Some mags don't change from centrifugal force acting on the dogs, instead their tripper retracts or in varying methods is moved away instead.

My two coppers for today.

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AS

06-06-2000 05:46:43




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 Re: Another John T Quiz Question in reply to John T, 06-05-2000 20:18:49  
I could be wrong, but does'nt spring tighten up and release to get a rapid rotation so the mag will spark. When the starter(or person)is turning over the engine there is'nt enough speed to get the mag to spark, therefore you need a short burst of speed. As for when the tractor starts, does'nt the centriphical force disengage the springs. I'm looking forward to your explaination John.



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H MN

06-05-2000 20:47:21




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 Re: Another John T Quiz Question in reply to John T, 06-05-2000 20:18:49  
John T: Ok. I'll bite. I normally don't post and don't know much at all about green machines but are you asking about a retard timing method used in mags? It should be held to full advance after it's started by weights. Before my accident to my ears last July , I really enjoyed seeing and hearing an old engine hand cranked. On some well maintained units one could hear the mag trip and the firing of that cylinder and then life to all cylinders...how sweet a sound that was. harvey

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add 2 that..

06-05-2000 21:20:52




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 Re: Re: Another John T Quiz Question in reply to H MN, 06-05-2000 20:47:21  
..... opps. In case there a dern lawyer reading these posts, I better add that a mag has a method of speeding up the rotation at hand cranking speed to that the voltage is higher for firing the plug. ( faster ya cut magnetic field the more EMF ya get) along with retarding the timing so ya don't get your arm broken from it kicking back. Many of the light airplanes have only one mag that retards and that one is used for starting and then both are switched on. (Mine were all Lycomings)....now I can get back in bed and go to sleep..I hope. Harvey

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