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Magneto Problem

bmetcalf

Well Known and Active Forum Website User
VOC Member
After several years of excellent service, my mag is not sparking at the front plug. The rear makes a good spark. I replaced the front wire when the terminal that grips the plug end shed a piece. The new wire shows continuity from the brush to the terminal at the far end. I cleaned the slip ring with a wood dowel and a rag & brakleen. Still, no spark, even with different plugs.

Any suggestions?
 

Magnetoman

Well Known and Active Forum Website User
VOC Member
After several years of excellent service, my mag is not sparking at the front plug. The rear makes a good spark. I replaced the front wire when the terminal that grips the plug end shed a piece. The new wire shows continuity from the brush to the terminal at the far end. I cleaned the slip ring with a wood dowel and a rag & brakleen. Still, no spark, even with different plugs.

Any suggestions?
quick reply from my iPhone. Have you checked to be sure brush is free in the pickup and not too worn?
 

Magnetoman

Well Known and Active Forum Website User
VOC Member
Addendum: swap pickups or try a spare in case present one has developed a short directly to housing.
 

bmetcalf

Well Known and Active Forum Website User
VOC Member
Addendum: swap pickups or try a spare in case present one has developed a short directly to housing.

Thanks,

New brush and I'll look at swapping, but the holders are "handed". I can also thoroughly clean the holder.
 

ernie

VOC Assistant Secretary
VOC Member
Annual maintenance task: clean slip ring; clean points; clean brush holders; check brushes; lubricate cam ring.
 

Magnetoman

Well Known and Active Forum Website User
VOC Member
New brush and I'll look at swapping, but the holders are "handed". I can also thoroughly clean the holder.
You'd think I would have learned by now not to write short answers to technical questions on my iPhone when I should be paying attention to the guy speaking at the meeting...

A quick, temporary swap of the pickups, aided by duct tape, could quickly point to whether there was an issue with a crack in the plastic that had filled with carbon. Cleaning it might (or might not) resolve the issue, but it will come back again if it is due to a crack.

Is your pickup Lucas, or an aftermarket replacement? The reason I ask is there are dodgy aftermarket parts on the market. The following post is my analysis of a failed slipring (I don't suspect your slipring at this point because you say one cylinder is getting a proper spark):

http://www.britbike.com/forums/ubbthreads.php?ubb=showflat&Number=507338#Post507338

On a personal note, I told you I would be free, but since then my day at NASA was taken out of my control by requests for individual meetings and a second seminar. So, no Vincent activities on that trip.
 

nkt267

Well Known and Active Forum Website User
VOC Member
I was talking to Stu Spalding the other day. He had a problem where the bike would not run on 1 pot he pulled off the other lead.expecting the bike to stop, and it chimed in on the pot that would not originally fire.Put the lead back on and it changed back.
He tracked this down to an after market slip ring that had developed a crack from 1 contact of the slip ring to the other and had filled with carbon..Changing the slip ring cured his problem..John
 

Kansas Bad Man

Well Known and Active Forum Website User
Non-VOC Member
Old plug wires can read continuity but when required to pass 30,000 volts, will leak to ground. In the garage on a dark night, with the lights turned off, start the bike. Then-----------. Swap the sparkplugs.
 

Magnetoman

Well Known and Active Forum Website User
VOC Member
Old plug wires can read continuity but when required to pass 30,000 volts, will leak to ground. In the garage on a dark night, with the lights turned off, start the bike. Then-----------. Swap the sparkplugs.
If you re-read his original post you will see that he is using a new plug wire as well as has used new plugs.

As just a point of information, only if a lead came off a plug at speed (or the earth electrode fell off the end of the plug), and nothing else shorted first (e.g. the pickup), would the wire ever reach 30,000 V above earth potential. At kickover rpm a magneto generates less than 5000 V. If it generated more, the engine wouldn't have to be kicked over so hard, which is why a fully magnetized magneto is a leg's best friend.
 

Kansas Bad Man

Well Known and Active Forum Website User
Non-VOC Member
A new plug does not always work. Get a couple of the hottest heat range plugs with the smallest center electrode, these plugs are of a low resistance. This is the best way for the garage mechanic to go if a weak magneto is in question. A plug that is fouled or inoperable meaning the emf is inadequate to jump the gap of the plug rather it is going to ground due to a fouled path. Your Vincent at high revs, say 5,000 rpm will have a voltage far in excess needed to give a blue to white spark at points of emf leaks. Your Vincent will at idle with a good mag
jump a half inch gap with ease, spark plug removed H.T. lead to ground. Checking the spark plug for spark at atmospheric pressure does not insure spark in real life due to the much denser insulating air between electrode and ground due to the compression cycle of the engine. A Vincent in good nick will be able to jump a gap of 2 inches in the atmosphere running on one cylinder at around 3500 rpm. Last point, a new spark plug wire does not insure that it wont leak. Never take things for granted. Again, put it in a dark room and see what is going on.
 
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