I'm not familar with that number. What brand spark plug?
I use NGK spark plugs. The EC250/300 should use a BR8EG (fine electrode) or BR8ES (standard electrode).
NGK also makes an iridium plug BR8EIX. This is what I'm using now. It provides a stronger spark for better burn.
Here is the NGK spark plug reference chart.
http://www.gasgasrider.org/NGKsparkplug_chart.gif
EDIT:
From the chart, it looks like "B10EGV" is as follows:
B = 14mm thread (correct)
no "R" = non-resistor type
10 = Colder than stock (GG recommended = 8)
E = 19mm thread reach (correct)
GV = Gold-palladium V-type electrode ($$)