Questions about Bendix SF14LN-3 Magnetos

[craigwadsworth]

Have a question about a Bendix SF14LN-3 rotor. How do you know if the internal rotor wiring or secondary capacitor is bad? We are not getting spark from the induction vibrator to the spark plugs. Symptom is the engine starts ok when cold but not when warm. Took the mag lead (and induction vibrator power lead) off the right mag and plugged in to the left mag and it worked ok (good spark at the plug). Right mag harness tested ok, all leads have continuity through the distribution block, primary and secondary coil check out, and magneto timing looks good. I am pretty much out of ideas unless it is a problem in the rotor.

[51fixer]

Common for a problem coil to degrade when warm.

[craigwadsworth]

The symptoms we see now are when the mag is cold.......

[John Dupre]

Have a question about a Bendix SF14LN-3 rotor. How do you know if the internal rotor wiring or secondary capacitor is bad? We are not getting spark from the induction vibrator to the spark plugs. Symptom is the engine starts ok when cold but not when warm. Took the mag lead (and induction vibrator power lead) off the right mag and plugged in to the left mag and it worked ok (good spark at the plug). Right mag harness tested ok, all leads have continuity through the distribution block, primary and secondary coil check out, and magneto timing looks good. I am pretty much out of ideas unless it is a problem in the rotor.

So did you make a mistake and it starts ok when hot but not when cold? Usually a starting vibrator bypasses the coil since the vibrator is generating the spark for starting and it passes through its own lead and set of points. If it start ok cold but not hot as stated then I would suspect the coil as 51fixer says. Very common for older style coils to break down. I have even seen it on modern ones. If it really does not start cold then it is possible the vibrator is weak and even if you can see sparks at a plug in the open there is not enough energy when under compression.