I have a 40hp 1974 Evinrude motor. It loses fire intermittently. I've replaced the ignition coils, checked the points & wiring. What is the resistance for the ignition coils supposed to be & exactly how do I check them? I have 2 different opinions??? Any other help will be appreciated. it has been running fine until about 2 weeks ago. It started miss firing (only on 1 cyl. I think) & now quit altogether???