At the time I first ran my engine (on a stand) I
tested the mag. terminal with a simple 12vt test
light and of course blew up the blub, so that works.
This was 3 wks ago. Anyways you can really feel a
difference running on mag. Then about a couple of
miles , pop , backfire went back to Batt. Back to the shop again with 12vt test light, it looks more
like 6vts. what I mean is the last time it blew out
my bulb. So I wonder what happened. Terminal is clean, and making contact. Magnets cant go dead
in two miles?? Guess I gotta live with it.
I had the same problem and it turned out to be a weak terminal post spring.
I have a 12 volt battery in my non starter T to help me get it going and then turn it to mag Once during a parade it would not run on the mag but it would run on the battery At first I thought it might be the switch but found a small piece of metal that looked like a piece of a cotter pin in the terminal post Once I removed it the mag worked great and still does. Still wondering where the metal came from.
In your case I would look at the post like folks have suggested and also check the wire connections
I don't see any way that the mag post connection could turn 12V magneto voltage to 6V. The source resistance of a good magneto is too low to allow that. I can believe a zero output magneto or totally intermmittent problem that is mag post related but I doubt seriously that the mag post itself is the problem in this case. A hot magneto can go weak instantly if the engine crankshaft has a lot of end play. When the crankshaft slides rearward the magneto gap increases quickly and the magneto strength drops off instantly. My son's 16 roadster had what we thought was a good magneto and it ran great for hours but then you could hit a big bump and the magneto would go weak and stay there. We checked the end play and found it excessive and in the process of checking it we pried the crankshaft back forward. The car would start find and run great on magneto again. I was amazed at how the crankshaft would stay in one place and run there for hours and then slip to another place and then stay in that new place for hours. We redid the entire motor while we had it out.
Another cause of weak magneto is the accidental application of car Battery voltage to the magneto post which will discharge the magnets. That can be done accidentally by a faulty ignition switch or a wiring short. If the battery gets connected to the magneto post for even an instant, that can do it. In those cases the magneto can usually be recharged in the car to restore the strength of the magnets but you must first find out why it happened since you can't recharge the magneto constantly as a final fix. Find the problem, install the fix, recharge the magnets if they need it but do NOT recharge the magnets just to see if that is what is wrong unless you have a good reason to know what happened.
Thanks John, I should have replaced the rear main
but I shimmed it instead .017. which is bolted to
the main cap. As far as the 6vt thing, it was just
a guess using a 12vt test light 1/2 the brightness
of 12vt. There is current but its weak.
John, I have a question regarding your statement "Applying car battery voltage to the post even for an instant will discharge the magnets". Could you explain how this is different than the in car recharge? Dont you apply battery voltage to the post to charge them this way?
And for anyone who may take this post the wrong way, its not meant as sarcastic or correcting John. I know John knows his stuff forward and back and am just confused.......
When charging the magnets in the car the magnets are carefully positioned to line up with the correct mag coils so that the applied DC voltage will strengthen the magnet charge. If you apply DC voltage when the magnets are not lined up correctly then you run the risk of decreasing the strength in the magnets of maybe even reversing the polarity of the magnets. In any event, any time you apply DC voltage to the you need to be very sure what you are doing.
Jim said it correctly. The assumption is that if Battery voltage gets connected to the magneto post at any random time, and for any random amount of time, the odds are against that adding any magnetism to the existing magnets.
The positioning of the flywheel is critical when doing an in-the-car recharge. This is why I always caution people who have a working magneto to not try an in-the-car recharge thinking they might make their magneto stronger when it is working OK. Lots of times the event becomes an in-the-car DISCHARGE of the magnets and the final result is weaker than before. In-the-car recharge should only be attempted when you have nothing to lose by trying it since the next step will be to pull the motor if it doesn't work.
I know opinions vary on this and the Ford literature gives conflicting guidance, but I'll go with those who say an out-of-car charge is better because the magnets get a stronger charge through direct contact and you don't have to worry about an accidental discharge.
Steve, you're probably right, but it's kind of like fishing--you can determine the phase of the moon, the barometric pressure, water temp, etc, but you've got to go when you've got the day off! Out of the car is best, but, I wouldn't pull the motor and tear everything apart just to charge the magnets.
As long as the weakness isn't due to a cracked magnet (or two).