Saving Old Seagulls

I know I mensioned it the other dat, but no responce.
You got this spark plug and the motor will not run, you put a new one in and it runs. I have tried to examin the old one.
1. it has the same electrical resistens.
2. check for connectios to ground, non
3. connect to a inition coil, result a spark
4. you block the gap with isolated material, and leave the plug without ground, then you connect to an ignition coil with high voltage, now you try with a grounded wire, if you can get a spark from the treads, I never could, so what is wrong ?
does the air/gasolin/oil mixture make moister on the dirty parts of the plug, so the spark goes the wrong way ?
any ideas ?

Hi Erik, I asked an electrician at work about your spark plug question and he came up with a possible answer. If the plug insulation is begining to fail then it may spark ok when it's not in the engine but when you try it in the engine the spark may find it easier to go to earth through the failing insulation than jump across the electrodes under compression. Electricity will always take the easiest route to earth.
Regards,
Steve