Magneto with heat effect.

[1KCBC]

Hi, Is it possible that the magneto to get worse in quality when the engine heat up?
Can anybody help please.

 

[fido]

Yes, I have that problem with my A7. You may need to get your magneto reconditioned but there is one easy check you can make first. When the magneto gets hot the contact breaker spring can expand until it rubs on the cam surface. The length of this spring as adjustable by means of a slotted screw hole at one end. If this is the case you can cure the fault by reducing the effective spring length.

[LJ.]

When the magneto gets hot the contact breaker spring can expand until it rubs on the cam surface. 

Ahh ha! Never thought of that when I was having mag problems earlier on in the hotter part of the year, especially during a long run. No wonder the insulation tape was gumming up and comming off.

Anyhow all sorted now as I shortened the spring so it does not touch the cam ring when hot.

Been a bit quite on here hasn't it?

[a10 gf]

Hello, have had a magneto heat problem as well. In my case it stopped working after some miles, as some heat related expansion in the armature made a wire loose contact when hot, and gave contact again when cold! Found it in the end, and resoldered it, it was at the capacitor side.

Regards

[fido]

My magneto came back from Tony Cooper this morning and it looks like a superb job. £154 including postage, which I thought was not bad as it has a complete replacement armature (the old one was sent back to me with the recon. mag, in a bag marked "scrap" !). The other magneto that's on the bike seems to be working OK at the moment although the A7 won't keep running at tickover. It will be interesting to see how she runs with the recon. one. It came with new pickups, leads and plug caps with a note to say only use rubber caps.

[Jon H]

I had a mag problem earlier this year, and although it wasn't caused by heat, it very nearly ended up producing a large amount of heat - ie a fire!

I thought I had a weak spark on the right cylinder and was trying to fault find.  On closer inspection of the magneto it became apparent that whenever I tickled the carb excess fuel dripped from the bottom of the float chamber on to the ht lead and then ran along the lead in to the lower pickup on the magneto.  When I removed the ht lead from the pickup, the core of the lead was soaked in petrol, and it had wicked along the lead for about 3 inches.

In addition, the pickup had a tiny hole in it and the spark was arcing on to the frame.

It's amazing that the whole thing didn't go up in smoke!

I've now replaced the ht leads and pickups and re-routed the ht leads away from any drips. ^-^

Cheers

Jon H

[LJ.]

Thanks for that warning Jon! Crikey I never thought that would happen, will certainly check my HT cables tomorrow morning.

By the way... Anyone carry a fire extinuisher on their bike?  probably not so funny thinking of the consequencies.

[a10 gf]

Yup, careful with those "normal" petrol leaks...I did ride quite some time with a less than well maintained 276 carb, think I must have been mad (probably just ignorant), lots of petrol on the mag, miracle it didn't all burn up. So now I am very strict about it, take no chances as I have used up my quota of luck in that dept.

I have two "english" heat (fire) stories from here, a Jaguar MK2 started burning in the middle of town, was saved because it was close to the fire department. Another lost his Ariel, he had just got it delivered from England, caught fire and burned 500 m from the ferry that delivered it from Newcastle!

Having some fire extinguisher available on the bike (in addition to a can of tire puncture fix) would be nice, but have not yet seen anything that was not far too bulky for the road. But at least in the garage, I -ALWAYS- have a checked & controlled extinguisher close by. On the road it's "only" the bike that goes, but at home? it may be the bike(s) + the car(s) + all the nice ol' tools & parts + the garage + the house +....!

Regards

[dpaddock]

Fido, you have given us a really good tip!
Thanks a bunch!

[bsa1960]

When the magneto gets hot the contact breaker spring can expand until it rubs on the cam surface. 

Ahh ha! Never thought of that when I was having mag problems earlier on in the hotter part of the year, especially during a long run. No wonder the insulation tape was gumming up and comming off.

Anyhow all sorted now as I shortened the spring so it does not touch the cam ring when hot.

Been a bit quite on here hasn't it?

I have this problem now. Did this cure your issue? I took the spring length down a few mm but don't hold out much hope. I'll take it for a ride later, and will report back.

Like you my bike would run lumpy after about 10 miles then stop due to lack of spark, adjusting the gap got me going again.

If this doesn't solve it I'll take it to my local expert.

[bsa1960]

That didn't work, but it's running the best it ever has, meaning it no longer conks out after a short journey, in fact I managed a good 22 mile outing this Monday. It's just not starting when hot.

I appreciate everyone's comments and will investigate along the lines you suggest.

[KenF]

On a Lucas ring-cam mag (such as a K2F) fitted with the later steel contact breaker assembly, the contact breaker spring is live, and so if it rubs on the earthed cam ring, it can cause electrical malfunctions. However, with the earlier brass contact breaker assembly, the contact breaker spring is (almost) earthed and so rubbing on the cam ring does not cause electrical problems, although it is of course best if it doesn't rub.

If a mag is causing hot starting problems, it is in my experience most likely to be a condenser problem, not so likely to be a HT coil problem, and even less likely to be a LT winding or dry-joint problem.

Ken
brightsparkmagnetos.com

[beezermacc]

I agree with Ken. Nine times out of ten it's the capacitor failing.
Andrew
www.priorymagnetos.co.uk