Sudden lack of any spark - 1965 AJS 16MS

[Old Thumper]

On Friday afternoon the weather was good so I thought I would take the bike out for its first run in several months. Bike started first kick and once warm would sit nicely at a steady tick over. The bike was running perfectly but after just a mile or so the bike suddenly stopped with an apparent lack of any spark. Bike was pushed home and left to cool down before checking again – still no spark.

Bike is 1956 AJS 16MS fitted with an SR1 Mag
Spark plug swapped for a brand new one – still nothing.
I have checked the magneto drive chain is intact and neither of the drive sprockets have somehow come loose IE the mag is still attached and spinning.
HT lead is in good condition and continuity checked – seems ok
Searching previous forum posts I found one that suggests a “quick” check of the mag can be done “Measuring from the body of the mag to the little pimple for the HT take-off to the plug lead , and you should see about 6000 ohms typically. If you see 'infinite' or a very high resistance, then the HT coil is open line”. Measured mine and it was returning a value of circa 5000 ohms, so that sounds about right.

Having trawled the Forums previous posts the only other thing that seems to get mentioned on a regular basis with regard to “lack of spark” issues is the Condensor/Capacitior. Due to that I have ordered a replacement condensor which is on the way along with a new set of points (might as well refresh it all whilst I am at it) and I am hoping that will solve the problem.

However, my question to the wise and sage out there; is there anything else you would suggest I could or should be checking whilst I wait for the new bits to arrive, to see if I can find where the Elektrickery is hiding.

[Mick D]

Hi

If your bike has a kill switch check that it or it's wiring isn't grounding.

Regards Mick

[jackstringer]

I know this is a bit late but, I had a rear brake switch act as kill switch. The wire for the brake light was touching the mody and so every time I applied the rear brake it would kill the engine.

I am just trying to sort mine out which is starting and running fine but cutting out after high revs for a bit of time (going uphill). I am replacing the Coil to be on the safe side.

[spookefoote1956]

Condenser gone knackered?

[56G80S]

I fitted a BrightSpark replacement for the condensor in my SR1 after problems with new aluminium foil type condensor.

They're a good swap and easy to do.

Johnny B

[Groily]

Searching previous forum posts I found one that suggests a “quick” check of the mag can be done “Measuring from the body of the mag to the little pimple for the HT take-off to the plug lead , and you should see about 6000 ohms typically. If you see 'infinite' or a very high resistance, then the HT coil is open line”. Measured mine and it was returning a value of circa 5000 ohms, so that sounds about right

The same resistance reading should appear between the inside of the plug cap and the mag body or earth. If it's open, there's a prob with the HT lead somewhere.
If the condenser had died I'd expect the mag to do at least something when it had cooled down, even if it wouldn't work properly. But, you never know, could just be that.
Sounds more likely though, assuming the HT side is good, that there is a problem with the low tension side, perhaps where the low tension live wire from the coil - and one end of the condenser - is attached to the side of the mag body half way up 'on the right' as you look under the cover. A short to earth there is always fatal. Or maybe there is a problem at the contact breaker as you suggest. Less likely (by far) is that the cam that opens the points has moved - but you can check if it's loose by wiggling. It's on a close-fitting taper and shouldn't come adrift!. A read of the Lucas instructions for the SR mag might provide a few more ideas - see http://www.brightsparkmagnetos.com/libr ... %20SR4.pdf