Magneto Advice please

[Alan]

The magneto on my 27 Chummy, a BTH, produces a nice heathy spark on a test bench, it runs ok on short journeys too. After about 20 minutes it starts to misfire and then the engine stops. It then wont start again. I'm sure its the mag because swapping to a coil conversion unit cures the problem instantly. This last happened Wednesday last week. This morning Ive had a look at the offending mag and its sparking again on all four. I suspect the condensor, but my magneto exoerience is limited. Can anyone point me in the right direction?

Thanks

[Bill Dixon]

Alan, Have a look at Bright Spark Magnetos web site,could be of some help. Best of look.

[Charles P]

I suspect that's condenser.
What are the points like?

C

[Alan]

Points don’t look too bad, but it’s not done much since they were cleaned.

[Charles P]

Whilst not being an expert condensers either totally fail, or degrade in a way which burns the points.
Not sure if totally fail can be intermittent

[Tony Betts]

Hi Alan,

I often borrow a friends car for the Scottish austin 7 club rally.

We had two mags that acted in a similar way.

Both mags had been professionally rebuilt recently, the condensors were new. And the new condensors were the problem in both mags.

They were both blic mags.

The new condensors didn't seem to be up to the job.

I've also been running Tracy's chummy in, and it has a similar problem, but at the moment it's to damn hot to go falt finding.

Tony.

[Robert Leigh]

I have BT-H on my Riley. It was fitted with an Easicap condenser by Brightspark magnetos. The big advantage is that this condenser is very compact campared with the original square type on the end of the coil, and it fits under the points. I have 2 mags done this way and have used Brightspark on 3 mags altogether. Bill Brown is a helpful chap, the only problem being that he is based in France (western Normandy). Sadly his UK based partner died a few years ago, but the widow still runs the commercial english end of the business and can supply parts.

[Bill Dixon]

Alan it is fairly common for capacitors(condensers)to be intermittent .

[Bob Olive]

As it starts/sparks from cold, it might be that the winding insulation is breaking down when hot - I've had several mags do this after a good start and then twenty to thirty minutes running when it starts misfiring, and then fails to restart. They did however start later when cold, with the same result after running for a similar time.
Try leaving it running on the bench using a lathe or drill with a jury rig full set of plugs for about half an hour or more. That might prove something.The safest thing to do of course is to get a (fully equipped) mag specialist to vet it thoroughly, though even they can be fooled sometimes...

[Robert Leigh]

Bob's comment is valid. On a hot day some years back I had a failure on the way to an event 50 odd miles away. I changed to my spare and called on the overhaul man on my way home, asking him to warm it up and test again. He phoned next day to agree that it failed at higher temperature. He had tested in his workshop but on the Riley it sits behind the upper part of the radiator, so I suggest that current hot weather may be a contributory factor.