Colin, black and sooty sparkplugs indicate too rich a fuel/air mixture, or oil bypassing the piston rings. Replace the sparkplugs with the proper type listed in the Owner's Manual and see how the engine runs. The spark you saw when testing the No. 1 sparkplug may not be strong enough to ignite the fuel/air mixture which would suggest that the coil is the culprit. What about trying a coil from another 300E to determine the condition of the coil?

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Fred Hoelzle