Magneto testing

Yesterday I came across this quick way to test magneto output.

As we all know just because we get a spark in free air it doesn't mean we will still get a good spark when sealed in the engine
So, get an old but serviceable plug and gap it to about 100 thou instead of usual 20/30 thou. This mimics the resistance when under compression. Connect HT lead and sit it on cylinder head. If you get a spark you're in good shape.

Multimeter is always my first check. Set it to 20kohms and measure from the HT lead to earth, anything 3-5kohms and you're good.
If it's open circuit you're wasting your time.
If noting at the HT lead check from the slip ring, finally if you can the coil itself.

Agree entirely Stuart. Just thought I'd toss it in for those without a multimeter.

100 thou - wow. Convinces me that my current problem ( weak spark post) with 3 thou gap is definitely coil especially when measuring 4ohms ! Surprised I get any spark mind.

Steve A wrote:100 thou - wow. Convinces me that my current problem ( weak spark post) with 3 thou gap is definitely coil especially when measuring 4ohms ! Surprised I get any spark mind.

Low resistance still smells like a short to me.

Woodsman wrote:

Steve A wrote:100 thou - wow. Convinces me that my current problem ( weak spark post) with 3 thou gap is definitely coil especially when measuring 4ohms ! Surprised I get any spark mind.

Low resistance still smells like a short to me.

I agree but my lawnmower measures only 6 ohm at HT coil and works fine. I'm thinking for some reason that measuring secondary winding resistance isn't as simple as HT to earth.

For old style coils it is that simple. My B&S powered Hayter from the 80s doesn't give a reading.
To completely rule out a coil remove it from the magneto and measure from the HT to the iron core.