the first thing i looked for was hair line cracks on the coils , you will find them on the back between block & back of coil spark jumps out through these cracks easy to see at night in the dark , fix them with epoxy solved the problem for me , secondly the rectifier the middle wire the red one gets brittle & breaks off difficult to see cuase it breaks off under the rectifier , i soldered another one back on coat with epoxy & cover with heat shrink , i have found 3 so far with this problem , clean all lugs & earth pionts , check every single wire for brittle ness & replace the coils have been coated with my epoxy formula