REFLECTOR: magneto question

sbjknox sbjknox at earthlink.net
Sat Jul 13 21:40:01 CDT 2013


----- Original Message ----- 
From: "Jack Prock" <jackprock at wavecable.com>
Subject: REFLECTOR: magneto question
> My problem is the mag I have seems to have a problem in that even
> when the switch is set to turn the mag on, it is still grounded. I pulled 
> the P-lead wire
> off the mag and did a conductivity test and was surprised that the P-Lead 
> post had
> conductivity to ground. Which means it is grounded all the time and would 
> not fire.

> Am I missing something here?
> Should the P-lead post be grounded all the time? I thought that was the 
> way to
> turn the mag off and on.

> I am going to run the engine again tomorrow, but I suspect the mag has a 
> problem.

> Does anyone have suggestions on looking at the mag?
> thanks,
> Jack

A typical Ohmeter will not tell you much because the internal resistance of 
the mag is very low and it is really not possible to distinguish between 
points open or closed.  That is why a "mag timer" is used.  It operates by 
recognizes the change in inductance from points open and closed rather than 
by change in resistance.

A relatively easy and safe way to check a mag is to pull the plugs (top or 
bottom, no difference) so that you can easily turn the engine.

1) remove one plug from each cylinder.
2) connect the plug wires from the mag you are checking to the plugs you 
removed and lay the body of the plugs against the engine, ie. make sure the 
outside of the plugs are grounded. It can be hazardous to the mag to let it 
fire without a plug load because the high voltage will instead find a path 
inside the mag to jump to ground.  This can cause the coil (inside the mag) 
to fail because of breakdown of the insulation.  Again, make sure all plug 
wires have a plug load.
3) turn the engine either with the starter or by hand and observe that all 
plugs are firing.  Switch off that mag and confirm that the plugs no longer 
fire.

With the plugs out is also a convenient time to check the mag switch (and P 
leads) and the impulse coupler(s) for proper operation.

$0.02

Sid Knox
Oklahoma

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