John/Malcolm:

As you know I have many tractors with the Bosch U4 on them, I have to say I have not had much problems with them at all. I have bought many of these mags at swap meets or on ebay and some are dead as a door nail but here is my secret for bringing them back to life.

If you have a dead one on the tractor take a magic marker and mark the coupler with two dots before you take it off the tractor so when you go to putting it back together just line up the dots.
Take the magneto off the tractor and set it on the bench or wheel or wherever you can find a place in your shop big enough to set it then chuck up a 3/4 inch socket into your drill and put a wire in one of the plug wire holes and loop it arround til it is about 1/8th of an inch from any metal part on the magneto.
Back the magneto up about 1/8th of a turn then pull the trigger on the drill quickly and let her rip fast enough so as to not let the magneto trip coggs catch. once it is rolling keep it going all the way wide open on the drill gradually holding the wire next to the magneto once it starts to spark pull it slightly farther away from the magneto.

The farther away you pull the wire the more the coil has to work and the hotter the spark will be, if the magneto is working like it should it should jump a spark up to 3/4 of an inch.
I have taken magnetos that were compleatly dead and brought them back to life by keeping the drill running for 2-5 minutes don't know how it works but I would swear somehow it recharges the magnets by doing this.

Now I know this will not work on all of them as some have bad coils in them but I would say it works about 9 out of 10 times expecially if you are working with a magneto that you know was good a year or two ago.

Just planted corn here last Monday May 14th and it is up two inches today!! The weather here has been awesome!!

Joe