Re: 1940 Johnson HD-15 Magneto testing

[joesnuffy]

Found this other thread here on our forums.

viewtopic.php?t=10117

"Can someone explain the theory in detail on how this point setup works with the carbon brushes? Found a .18 mfd condenser but still can’t get it to fire."

Then Response

"I’ll be daring and give it a try. You have an alternate firing two cylinder with the coil from an opposed twin. Testing the coil is done like an opposed twin where you test across the plug leads if you have the flywheel off – not lead to ground like a normal alternate firing twin. When everything is working the brushes ground alternate sides of the coil. To do that on the underside of the flywheel is a bakelite disk for an insulator. There are grounding points in it. The brushes make contact on each side and provide the alternating spark. The brush holders have to be clean and provide insulation for the brushes and there has to be contact through the grounding points with the flywheel. Once everything is clean and correct magic happens.
Did that help any? Experts don’t be afraid to jump in here."

Here are my thoughts below:

Watching your video when you tried to ground each plug it was like no real definite reaction like a cylinder that isn’t firing seem like both weren’t firing evenly their firing sporadically or one is double firing.

Have you checked the flywheel bakelite for cracks. Possibly rig/attach like an omc high tension side coil wire to flywheel using your stevens meter and check it all around bakelite for arching with your probe?? Also check the metal ground point on flywheel for arching I am guessing it should arch in that spot??

Thinking to myself what I would do. What if you use a timing light on engine. With magneto in a fixed position where motor will run. Make marks on flywheel and engine for both cylinders where points open so with the light you could see if while its running if your getting double fire on a cylinder instead of single fire?? You could make the marks different colors or a big number 1 and big number 2. If a cylinder is double firing that’s a brush problem but it might help you identify which brush. Also if its double firing the brass tab in flywheel might not be grounded correctly the stevens meter probe test should tell if it is or not.

Joe

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