A point of clarification Freddy. Your spark plug can only spark if the body of the plug - the metal part with the thread - is grounded, say to the cylinder head or cylinder. Whenever you are in doubt about the ignition system, it is a good idea to check the spark directly from the end of the plug lead to the cylinder head. If you get a fairly strong spark that way - it had better be blue and at least 3 mm - then you have ignition but may well have a bad spark plug. Remember, 2 stroke spark plugs are poisoned by the oil in the fuel, and may have a weak and unreliable spark after about 15 hours' use. If they've done a lot more than 15 hours, even cleaning them is only partly and temporarily helpful. (The situation is quite different with 4 stroke spark plugs - I tend to use them until the electrodes are eroded enough to change the heat range - but I have to file the contact surfaces every few years, because they develop cracks and cause high resistance.) Also, it is by no means uncommon for new spark plugs to be defective. You should always check the ignition with a known good spark plug, and a new one is not known to be good.
I don't like to put the spark plug close to the open spark plug hole to check spark with 2 strokes. Unlike a 4 stroke, the 2 stroke will expell unburned fuel from the plug hole each revolution. With the spark plug sparking close by at the crucial moment, this can cause an accident that might (with a fair amount of bad luck) hurt someone or cause a grass fire.
