It's not the spark plug but the spark voltage, f the plug does not fire in open air with TWICE the gap (0.040 +) it's not likely to fire under compression at cranking speed.At speed the engine can keep running because the spark voltage is higher but once you shut it off,it won't start up again.I think a lot of CD units fail due to moisture penetration,which causes the HT transformer to gradually fail.The connection to the plug wire is the weak point.(RTV !! )Mount it with all leads pointing down& to the rear and seal with RTV.Once moisture gets in it will never ever get out again.The reason coils fail is that water has a very high dielectric constant (81).this means that the electrical field is concentrated in the smaller gaps between winding layers.The insulation starts to break down progressively& conductive paths become established (tracking) reducing the spark output.