Johnson 25 - spark jumping from boot

[boatdontgo]

I'm running a mid 1970s 25 hp Johnson. On the top sparkplug, if I push the rubber boot right on to the plug, so the HT wire connects to the plug, it doesn't fire and the motor runs on one cylinder. If I pull the boot off and have it sitting just on the end of the plug (not attached, just the rubber dangling on the end of the plug) then it runs like a champ. What would cause this and how do I fix it?

is it likely that the boot is conducting electricity and when it is pushed right onto the plug it is close enough to the block to short to the block. And conversely, when it is pulled off the plug it is too far away from the block, so the spark jumps to the tip of the plug?

 

[fdrgator]

Two things:
1. With the boot pushed all the way on, spark is arcing to the plug's metal part. If so, you probably will need a new boot because it has burned a carbon track inside.

2. Bad or fouled spark plug. Making the spark jump that long gap before getting to the plug is forcing the voltage up, and the higher voltage is firing the plug regardless of the defect.

Try a new plug first.