concerning magnetos : do they suffer from failure due to bad earth like corroding of mounting joints? There is a brass screw on the side of it which i am told is earth related, can this cause problems, corrosion, need readjustment etc. Everything else I have cleaned, redressed points, cleaned HT lead at both ends. The screw in spark plug cap was rusted and difficult to clean would this perhaps stop the spark? It was sparking like a good 'un before I laid it up for 4 months over winter but it is outside under cover although I know damp finds a way in, could this be the issue? Help greatly appreaciated.
Unlikely earth issue (unless plastic shims between mag and platform. To eliminate this question take a 2.5mm wire from the brass earth lug that holds the mag end on with grub screw to grip the wire and a ring crimp to the nut on the crankcase below the barrel.
Next you may need to ensure pure copper HT lead and non-resistor plug cap and non-resistor plug. Make sure the brass washer that the HT lead goes through is corrosion free (I solder mine). Check that the HT pick up and earth lead are clean and not worn away.
How old is your magneto?
Next weak point is an original paper/oil capacitor, but that will need a strip down.
By brass screw do you mean the earth brush? Unscrew this carefully and check the brush is free and the spring is still "springy". Leave the HT brush holder off and try blowing some warm air in as it may have condensation inside. You can clean the brush tracks with a bit of rag and a bit of wood (kebab sticks are ideal) while turning the engine over.
Check the screw in the plug cap is actually connecting with the copper core, its possible to screw the lead in and miss it.
My ZB32 often has no spark after laying up over the winter. All I need to do is clean the contact breaker points. I use a small piece of medium grade wet & dry, 150 or thereabouts. I prise the points apart and insert the wet & dry between them then pull it out with the contact breaker spring applying the pressure. Turn the wet & dry over so that the abrasive side is facing the other half of the points and pull it out again. Job done. Spark restored.
That's my experience too. I've also had some corrosion on the points blade to the all points base.
My Velo always needed the points cleaned after the winter lay up.
thanks plenty to try there
Sounds like answer is to ideally not lay it up😂
Having said that I used my 'scrambler' to go to work yesterday (last run on it was Boxing Day). Fired up second kick. It did stall in traffic jam on way home in the evening requiring a tickle and three kicks (sticking float needle or old fuel. Maybe I need to check my points are clean too!!
Good luck with your mag.
It's all rather strange. My B33 and my Shooting Star always start after the winter lay up. Started my B33 today. But my Velo was always difficult.
Easily answered..Velo's are thoroughbreds and hence need additional love and attention.
Should give the Velo fresh grass instead of hay. Might behave then.
I had a Greeves 250 Challenger that had an intermittent spark. I had cleaned the points, and still poor spark. I then tested everything with an Ohm-meter, and found that the riveted tungsten tip was so corroded with the spring that it was often open circuit.
So maybe if you have a bike with contact breakers over 50 years old, maybe it's a good idea to buy some new ones, even if you have polished the gap to a mirror finish.
Repro parts being what they are, I'd take my chances with the 50+ year old items rather than buy new.. ;)
Quote from: V500 on 01 Mar, 2026, 21:18 Repro parts being what they are, I'd take my chances with the 50+ year old items rather than buy new.. ;)
Based on experience i am inclined to agree (preferably NOS tho'!!)
Quote from: V500 on 01 Mar, 2026, 21:18 Repro parts being what they are, I'd take my chances with the 50+ year old items rather than buy new.. ;)
Contact breaker points used to be platinum, then they were platinum plated. Current repro ones are often nickel plated copper. Once the nickel wears through they last no time at all.
I recently bought a mag points cover that needed a good half hour of filing before it fitted. Most frustrating was an electrical part of a car, I wanted to shorten and re-solder the connections (they were crimped). Could not get the solder to stick, turned out the wires were copper plated steel, what hope it there?
Several suggestions and points showed up to be problems. The brass screw on the side of of the mag was corroded at the tip, put on new HT lead and cap which had gone rusty inside, cleaned the mag contact, reassembled and then got a lovely spark! Now the carb is leaking and won't fill properly which i suspect is old eco fuel i stupidly left in over winter. It's probably gunged up the jets and certainly the felt washers are also leaking. HOWEVER, I feel confident that after I get a kit of gaskets, clean all the bits and reassemble the carb the M21 will roar into life at the age of 71. Thanks for the help everyone.