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Magnetos, Ignition Coils and Spark Plugs Discussion about magnetos, buzz coils, spark plugs, ignitors and low tension coils.

Magnetos, Ignition Coils and Spark Plugs

Caterpillar D25U with Bosch MJK Magneto

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Old 02-10-2012, 07:52:25 AM
sleeve sleeve is offline
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Default Caterpillar D25U with Bosch MJK Magneto

I have A Caterpillar D25U with the rope pull start arrangement commonly used on these machines. In the span of A year I have had to remove the Bosch MJK Magneto and take it in for service 4 times. The latest episode is A timing issue, pre detonation and the rope recoiling . Painful and frustrating. Points, plugs , wires are all ok. The issue seems to be the contour of the cam the points block rides on. Once my mag guy had to rework the countour with A file. When I get the mag back from old Mike and install it , all works fine. Starts require only A few pulls. Then , invariably the problems start. The machine is kept dry, stored inside much of the time , is very meticulously maintained as it is A hobby in addition to being A workhorse on the property. There must be an answer somewhere to the unreliable nature of the mag.Other mowers , old machinery I own are left out in the elements for months , yet start right up with fresh gas, A clean plug and some choke and throttle tinkering. So, what is with the Bosch mag. and the Caterpillar? What am I doing wrong? Is it characteristic for the thing to be so tempermental and unreliable ? I have decades of experience with heavy trucks and equipment. Had A D82U some years back that even when the pony was hand cranked, never had the problems this little one has, and never the magneto. Any suggestions ? Feedback ? I want to keep the machine as is , so I avoid the usual electric start suggestion. Trading deck chairs on the Titanic .........Sleeve
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Old 02-10-2012, 11:49:39 AM
Andrew Mackey Andrew Mackey is offline
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Exclamation Re: Caterpillar D25U with Bosch MJK Magneto

Is the points rubbing block wearing out, changing the points setting? Changing the profile may have caused a wear issue with the points. Also, if the cam surface is not absolutely smooth (read polished), it will wear out the points rubbing block prematurely. Also, is there a felt lubricater on the mag? usually there is a felt that provides oil to the face of the cam. Sometimes a touch of grease is used instead. It sounds like you will have to either replace the cam in the mag, or replace the mag itself, unfortunately. Perhaps a later version or a different mag will work better in the long run, if one is available.
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Old 02-10-2012, 12:47:03 PM
mmcdonald mmcdonald is offline
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Default Re: Caterpillar D25U with Bosch MJK Magneto

Those are fine mags. Might want to find out what your mag man has had to do to the mag on other trips besides the trip where he filed the cam. This would probably lead you to the actual problem and then it could be remedied. The only generic problem mags have usually from being unused outside is corrosion on the points. If this is the problem you could clean them up yourself. Good luck.
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