1930 Big 4 - Spark Plug Colour??

Put the old one back in ,before you do some damage.  If the old plug then begins to look like the new  ,check for restricted fuel supply/wrong ignition timing . its too weak/hot . If the old plug carries on like it did before then it looks great to me!. The new plug must be too soft . Bin it. 

Considering you would be running on unleaded (my point is you won't get a grey or light tan electrode on unleaded).

Both look ok to me, considering the New one had only done 10 miles. 

See what it looks like after a couple of hundred miles.

BTW my BSA M20 ran for so long on the same plug the nut had become as rusted as the cylinder head. 

I 

Green Spark Plug website has lots of useful info on plugs.  As far I remember - if a plug does get too hot, the hot plug itself is not a problem for the engine but only a problem to itself.  It looks OK to me but as Don says all those colour pictures showing how to read a plug don't work with unleaded.

Hi David,

   As Robert says your engine/new plug is running too hot. Those marks on the plug body are caused by spontaneous detonation otherwise known as pre-ignition or pinking, although you may not have noticed this with general engine noise. The marks are a mild version of spark erosion as the plug is acting like a glow plug in a diesel engine and igniting the fuel before the spark. You might want to retard the ignition a degree or two, try using the advance/retard lever first to see if the bike runs better.

As Don says the colour of the  insulator  does not tell us much now.  But look closely at the corona line on the old plug .  Pretty good.  The new plug corona line looks to have has gone towards the body , or its  too hot  to leave a deposit,   . Also should be seeing a light ring of carbon  nearly all round the plug mouth.

The question should be,  whats changed?,  if its only the plug, then  change it back.,and check what happens  Are both plugs the same grade and make ?. If  yes,then   something else is different ,perhaps the  way you rode?,  old petrol / new petrol?  , Light use/ hard use.  Eo petrol/ Eo5 petrol.  If you put a new plug in a hot engine and check it after a short  hard ride its going to look very different to a used plug  after a short  gentle  ride from a cold start.  I keep meaning to take my bikes to a  guy with a dyno facility  and exhaust gas  reader ,  has to be an interesting  experience. To effectively tune an engine you need to use a set of fixed parameters and then just make one change at a time and check the result. Best to have a few new plugs for each change as cleaning plugs is not reliable.

Too far advanced causes detonation which seems to perhaps be your problem, and too far retarded causes engine overheating and sluggish performance.  Is it possible that the timing has slipped?  It's easy to measure piston drop at full advance in the usual way on the side valve engine.  No need for a degree wheel.