Author Topic: Beware those mag. brush holders!  (Read 4677 times)

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Offline george sutton

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Beware those mag. brush holders!
« on: 25 Jun 2010 at 04:48 »
For those who have followed the problems of my T35 missfireings, and all the work done to cure it, i want to make you aware of what i have just discovered.    with absolutely no more options left, i pulled off the magneto brush holders one more time to check brushes and springs.  BUT; this time with the holder in my hand and in a different light, i saw a 1/8"high 'pin' on the brass contact face.! see photos. Here then must have been the reason for ht/leads coming loose, missing , and failure from road vibrations.     lathe men will know what i mean when i say it was a 'parting off nib'left on the contact when made, and normally remove later. both holders had these left on . why didn't the makers spot them? why didn't the co.,who rebuilt the mag two years ago spot them? don't know.   but on the bike,  and looking directly in to the holders, end on to contact,they are not noticeable.       Rogue nibs now removed, engine starts fine cold, begins to miss as it warms up, then fails, and refuses to restart when hot.   obvious now then that after all this time of trying to run like this ,that its broken down the mag .windings.   so there you are, be careful what you wish for when you want to recapture a little of your youth.    Am going to contact the co. in Cornwell that rebuilt the mag,asking for there comments  .   Denis s.


« Last Edit: 26 Jun 2010 at 10:22 by Dave »

Offline Brian Cape Town

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Re: Beware those mag. brush holders!
« Reply #1 on: 30 Jun 2010 at 20:30 »
Most interesting!! A couple of years ago I finished restoring my MkV here in SA having had the mag rewound in the UK. Initially all was well then as you experienced the dreaded misfire, and then no fire!
After the usual points, plugs, timing etc. I changed the new brush holders in the mag and fitted the  old ones I fortunately still had. Problem solved and still going strong, tomorrow I will look at the removed brush holders to see if they have the "pip" in them.
Hope you now finally get going.
Brian.

Offline george sutton

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Re: Beware those mag. brush holders!
« Reply #2 on: 01 Jul 2010 at 19:01 »
hello Brian, my your a long way off. certainly seems like you had a problem on the holders, if not unwanted 'nibs' then tracking. glad its ok now.  I sent this defect to the gentleman who rebuilt my mag over two years ago. within hours of receiving my letter he was on the phone  to ask me to send it to him. down in cornwell .uk. mr.winter-baker has been brilliant  over this. he has tested the mag for hours while heating it up to check for failure on the output. He found no fault ! but today, he has stripped the armature, renewed the capaciter, and the slip ring , and again tested it almost to destuction . a brilliant service indeed!    but it will leave me going back to carburation , which i have been over four times already.     As the months roll on into the first  year of the cursed problems of this m/c, is it really possible to own a bike that is truely AFRAID to venture onto the road. I now Eddie, you will say rubbish !  but my faith is really getting bashed here. wheres that blasted tool box, ?


Quote of previous post removed - Dave, 2nd July 2010
« Last Edit: 01 Jul 2010 at 20:58 by Dave »

Offline eddie

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Re: Beware those mag. brush holders!
« Reply #3 on: 06 Jul 2010 at 20:55 »
Denis,
          Your latest postings beg the question "what plugs and leads are you running on"?  The best setup for use with magnetos is: non resistor plugs gapped to .018", unsuppressed caps and copper leads. Anything that further increases the resistance can cause the windings to break down or short out. It is also bad practice to check how good a mag is by seeing how big a spark it will generate - this also can cause internal shorting across the windings (BTH fitted 'shorting screws' to limit the length of spark and prevent shorting across the windings).
      NGK B7HS plugs seem to be the most suitable for postwar Douglases.
      I very much doubt that the 'nibs' on the pickups were the cause of your problems - they would be more than capable of transmitting the power generated by the mag. Also, the fact that they were still in place meant that they had not broken off and created a larger gap for the spark to jump.
                                   Regards,
                                                Eddie.

Offline Doug

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Re: Beware those mag. brush holders!
« Reply #4 on: 07 Jul 2010 at 00:24 »
The nibs might have been intentional. A number of HT leads fittings have barbs or spikes to pierce the end of the cable center to improve contact.

-Doug

Offline george sutton

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Re: Beware those mag. brush holders!
« Reply #5 on: 08 Jul 2010 at 06:55 »
Denis,
          Your latest postings beg the question "what plugs and leads are you running on"?  The best setup for use with magnetos is: non resistor plugs gapped to .018", unsuppressed caps and copper leads. Anything that further increases the resistance can cause the windings to break down or short out. It is also bad practice to check how good a mag is by seeing how big a spark it will generate - this also can cause internal shorting across the windings (BTH fitted 'shorting screws' to limit the length of spark and prevent shorting across the windings).
      NGK B7HS plugs seem to be the most suitable for postwar Douglases.
      I very much doubt that the 'nibs' on the pickups were the cause of your problems - they would be more than capable of transmitting the power generated by the mag. Also, the fact that they were still in place meant that they had not broken off and created a larger gap for the spark to jump.
                                   Regards,
                                                Eddie.
  morning Eddie.all you say about the leads was one of the first things i did at the begining of this saga, plugs are NGK B6HS. Mr. Winter-Baker of bodmin. has rebuilt the magneto and tesed it out to pump out 20,000volts running Very hot. got it back now,hope to refit today.mr. w.b says the nibs shouldent have been on the holders, but your right , that voltage would have jumped easily. So, we could still have a fly in there somewhere. might even write a book on , The bike that got its on back on man for leaving it laying around in bits for so long! going to look for the fly swat now. Denis s

 

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