Spark Plug Issue

[dr.j]

Anyone come upon a spark plug that will not fire when grounded to the engine block yet fires a solid blue spark when the tip is within half an inch of the block and the body of the plug is not touching any metal?

 

[J Franklin]

the spark takes the easiest and closest path to ground. I would guess that it is arcing internally normally but when held tip to block it must take that route. Clean the plug cavity thoroughly and see if that doesn't solve the problem.

 

[Tom Wesenberg]

Yes, that's caused by a fouled plug. The last one I helped my neighbor fix like that I used a propane torch to heat the center and ground electrodes red hot, then his small engine fired right up.


When the center electrode gets wet or soot fouled it has lower resistence and the spark can bleed off to ground before it jumps the gap. When you hold the plug wire away from the spark plug some distance then the coil has to build to a higher voltage to jump the combined gaps, and this almost instant high voltage jumps the plug gap before it has time to bleed to ground.

 

[dr.j]

Gents, my question has been answered and I thank you. The plug was very wet and now I know why it was doing it.