Correct plug?
 

My 300sm has a B10EGV in it, is this correct?

That seems a bit cold to me, but, if you ride the bike
hard on the throttle in high in the RPMs. That just
might be in the ball park.
Let's wait and see what the real knowledgeable folks
here have to say. As I am interested in stuff like this
also.
Jeff So. Cal. USA

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 ($$)

For some reason I would "flame out" or foul every BR*** plug I tried. Back then my kid rode slow and it always happened after long slow downhill sections. Pour the power on and "poof"! I now run B8EG, same plug now for 7 years. (yeah, I'm cheap:D ) The non-resistor plug has worked well for me.

I always thought the # in the middle, ie 2-10 refered
to the plugs heat range.
This is where I got that idea from.
http://www.kaila.net/tl125/tl125ngkcode.html
correct me if I am wrong. thanks.
Jeff So Cal. USA

Oops! you are correct. I read the NGK chart wrong. I editted my previous post. Thanks.