Click this link:
http://www.benchmarkworks.com/articles/tech/hardstart.html
In addition:
The resistance across the two spark plug leads on the ignition coil is usually 11 – 12 K Ohm (original coils and new “made in Germany” coils). Some after-market coils have a resistance up to 20 K Ohm. When the magnetic field of the rotor is still OK, this higher resistance will not necessarily cause a problem. A weak spark at the spark plug can have different causes: a broken or poor quality ignition coil is one of them. When the insulation (schellac) within the coil is damaged (crazing), the internal windings will short circuit, especially when the coil becomes warm/hot. This will cause a drop in voltage output. (and the fact that an increase in the coil temperature always leads to an increase in resistance). This cannot be measured with simple DIY equipment, but difficulties with starting when the engine is hot, is an indication of this.
Also a weaker magnetic field of the rotor will lead to a lower voltage output. The magnetic field of a rotor can be strengthened by specialized companies.
It’s recommended to use spark plugs and spark plug caps with 0 Ohm resistance.
Pros and cons of Iridium spark plugs:
Iridium is a metal that is harder and more durable than platinum. More importantly: these spark plugs have a small center electrode which means they use less voltage to generate the electric current.
On the other hand, it is said that using a platinum or iridium spark plug in an older engine may lead to the fouling of the plug rather rapidly because the plug is too cold to burn off any excess fuel and oil that get into the combustion chambers of the engine. Burning off extra fuel and oil is what the hotter copper core spark plugs did very well. (source: www.yourmechanic.com)