77 Evinrude 6hp ignition problem
 

Model 6704, L81903
Status: rebuild
Compression: good
Plugs: new - #1 poor spark/#2 no spark
Switch Box: new
Coil test: Primary/Secondary - both 0.02-3/300-350
Sensor Coil: 40/no shorting to ground
Charge coil: Not tested.
When plugs reversed, backfires thru exhaust.

S-80 neon tester required to test charge coil, & have been told I can
build one. Does anyone have a diagram?
Also, under circumstances, what is possibility of flywheel being
faulty?
Tks, alp