Vintage BMW Motorcycle Owners

Hi Everyone
I've got a problem with the magneto in a R69S!. Intermittent spark: when I say intermittent, it can sometimes run for several minutes then stop. Sometimes I can come to the bike and there is no spark at all. Hours later, without touching anything, the spark will return for a while.
I've changed the coil (blue one fitted now), points and condenser. I've even checked the rotor/stator airgap. Rotor/stator timing to 'S' mark is spot on. Points gap 14thou. Good insulation around the bolt connecting the points to coil/condenser.
Ignition switch earthing lead disconnected at the coil. HT leads show 0.5ohms to plug cap. New and correct plugs fitted.
I'm at the point of throwing the towel in and fitting a VAPE system - but thought I'd check in with some of you good folks to see if I'm missing something (except hair!!).
Thanks in advance for any info.

Welcome to the forum! Are you familiar with the magneto safety gap? The two spark plug leads attach to the forward side of the coil. Just below those two attachments, on the magneto body, are two "points". The gap between the ends of the plug wires at these points should be 10-11 mm. If closer than that, the spark could jump to the these points instead of the plug...the spark tries to find the easiest path to ground. If larger than that, the magneto could be in danger of damage if for some reason the spark plugs or wires fail and the energy can't find a way to ground.

Your rotor may have lost it's magnetism. However. have you run an unwaxed card through your points, even though they are new? Dirty points cause these problems.

Dave

Can you give us more background? Does this happen all the time since you got the bike or did it start up after some maintenance?

Hi All
Many thanks for speedy and inciteful replies. Much appreciated.
'safety gap' checked and spot on.
..'Points' operating perfectly - checked by disconnecting coil and condenser terminals.
Bike ran, but not up to spec. when acquired. Carbs, pistons and valves checked: all good. All 'points' pointed to ignition problems - hence replacement of coil, points and condenser. Bike ran OK, but not outstandigly so for a while. Laid up for winter and these problems became apparrent during spring recommissioning.
I can't rule out rotor problem - but magnets seem very strong. Air gap to stator <10thou.
Is there any way to test/check the coil?.

A good discussion magnetos recently:

viewtopic.php?f=7&t=18153

Disassemble complete mag body and clean all contact surfaces to get a good ground throughout. If you have everything correct,these mag systems are practically faultless other than coils.

And don't forget leads, caps and sparkplugs. These can be faulty too. And a cap with 5k resistance is not a good idea either. Best are caps with no resistance or 1k Ohm max. The same for sparkplus. Don't use sparkplugs with build in resistance.

Have you eliminated the possibility it is just the wire that runs between the points and ignition switch.
When the wire is disconnected at the points the engine should never shut off.

Don

Done all of the above, folks!. Points operating correctly (inc. connecting wire). 0ohm resistance in both HT leads. Correct plugs and, as already stated, kill wire to ign. switch disconnected.
Really doing my head in.
Only thing I can think is new coil has gone faulty - but buying another one just to check is not really on, as cost here in UK is 30% of cost of VAPE conversion.