Lawn mower spark starts only when I hold plug wire

[bacarlson]

This one has me stumped. My rider fires and runs fine only if I have a spark plug gap!

My mower rider, an MTD, starts fine and runs if I hold the plug wire about 1/4" from the plug. Good spark, all that. If I actually connect it to the plug, it dies nor will it start if the plug wire is clamped to the plug.

The only quirk here that I can think of is that I left the key in the "on" position all night....if that makes any difference.

Bruce

[Eric in North TX]

The only time I've seen that happen is when the spark-plug is fouled or bad. The act of holding it away creates a secondary gap the intensifies the spark and lets / makes it fire in spite of too much resistance in the plug.

[bacarlson]

Well, that was it! Geez....a fouled plug should be the first place I should check but I didn't know the scenario I described was even possible with a fouled plug......

Thanks Eric..... tonight we get to cut the grass before we leave now for the Labor Day weekend! :)

Bruce

Eric > The only time I've seen that happen is when the spark-plug is fouled or

[Jeff Wisnia]

I've seen that too, but just to be a PIA, I'll correct your language by saying it's too LITTLE resistance in the plug caused by fouling, not too much.

That lowered resistance prevents the coil's secondary voltage from building up high enough to spark across the electrodes, but as you correctly stated, the "extra" external gap lets the voltage build up until it breaks down, which connects much of the full secondary voltage across the plug gap.

Change the plug, I bet you'll find that fixes the problem.

Jeff

[Eric in North TX]

I'll give you that one, makes sense to me ;)