Re: 1975 johnson 25hp keeps trashing primary
This is getting out of hand. The 1975 25hp has the "Low Tension" magneto. It has a single primary coil under the flywheel with two sets of points and two condensers. The primary coil generates electricity for both cylinders. The spark coils are mounted externally, on the cylinder head. It is the primary coil burning up that we are trying to solve.
Now, 12V is taken off one side of the starter solenoid, from the post where the battery cable goes, and goes to the "B" terminal on the ignition switch. That provides power for the choke and any accessories such as a tachometer. The Choke connects to the "C" terminal and is activated when the key is pushed. The magneto connects to the two "M" terminals, or in some cases they are marked "M" & "G". Those two M terminals should never see 12V. "S" terminal activates the starter solenoid and is activated only when the key is turned to start position. The Acc is for accessories such as the tach and has 12V when the key is turned on.
The key switch connects the two "M" terminals together when the key is turned off. That shorts out the points and kills the igniton, stopping the motor.
There is no alternator or battery charging on that model.
Now that everybody has a crash course in Low Tension Magnetos 101, lets try to figure this thing out.
When the problem was first presented to us, everybody immediatly assumed that 12V was being applied to the coils. That was a reasonable assumption, because it happens so often from people using the wrong switch or wiring it incorrectly.
I am trying to keep mdcwells from going down the wrong path by checking everything under the sun. All that needs to be done is put a voltmeter on the M terminals, one at a time and ground. Note that there is no ground wire provided, unless there is one for a tach--I'm not sure about that. With the key on and/or the choke pushed, there should be no voltage at the M terminals. There will be pulses if you crank the motor, and I'm not sure what the voltage will be.
OK, if you are not getting 12V at the M terminals, we guessed wrong and that is probably not the cause of your coil problem.
HOWEVER, there is another possibility. If somebody was installing an accessory and was looking for a ground wire, and saw that the M trminals were grounded through the black wire, and said AHA!, here's a ground, and used that to ground their accessory---AHA again--he just applied 12V to the ignition. Actually 12V, dropped by the result of the resistance of the accessory, but that's tech talk.
If none of this proves to be the problem, you got me stumped and I don't know why you are melting coils.
Sorry about the long post, but this needed to be cleared up.
