I had a similar problem with my B while mowing. Acted like someone hit the kill switch on the mag.
I messed with it a while, checking what I thought was everything and finally gave up and took it to Frankie Lynch over in Moody, Tx. Frankie must be 85 years old and can fix nearly any part of a mag.
He cleaned the points, found that someone had left out a washer under the screw that holds down the points spring and she started right up! Lesson learned: check the simple stuff first and spend time looking at the details. The missing washer allowed the screw to extend too far and was keeping the points from opening fully, causing them to burn just enough to not make good contact.
After the fix it would shoot a hot, blue spark over 1/2".