Spark Plugs

Everyone has their opinion of what plug to use or what works best for their setup.  Mitsubishi installed originally a plug that is most likely one you have never seen.  Common plugs are NGK BPR7ES or perhaps 6s for non intercooled cars or 8s for a highly boost car.  The plug was NGK BUR7EA-11.  This information is in the factory service manual in section 8, electrical. 

The breakdown of this part number you can see from the chart below.

BUR7EA-11 from the above chart is surface or semi surface and that is a completely different type of plug from a BPR7ES-11 because that you can see has a more projected and less protected insulator.  Rich air/fuel conditions, rich idling, leaky injectors, stuck injectors, wrong gap all those can contribute to foul that projected insulator.  A inadequate oil separator will eventually cause carbon build up in the cylinders.    Fouling can mean no start, hard start, high rpm misfires.  Wrong plug wire routing, tangled wires, wires shorting out to the engine, deteriorated plug wires or boots can also cause misfires.  Those plastic clips that were on the valve cover and held the wires separated and in place had a purpose you should retain those and use them.  Unexplained bucking or popping under boost at higher rpms could be misfires. 

This is the explanation of the type plug Mitsubishi installed in the engine when it was new:

Surface and semi-surface discharge firing tips are less sensitive to voltage requirement increases due to gap growth(wear). Note the concave cut on the ground electrodes for even Gap growth (wear). In addition, this type of construction aids in burning away the carbon which can act as a bridge between the center electrode and plug shell. This bridging can cause plug misfire.

Here's what they look like, they come gapped at .044.  These plugs are harder to locate but are available at Summit/Jegs etc. 

Unported combustion chamber

Ported, more to the shape of a hemispherical chamber.