Your bike is running rich. Then engine is getting too much fuel for the amount of air. This is not the same as having too much oil in your fuel. From your description and photo it seems your plug deposits are dry, black and sort of fine fur / velvet textured - all indications of excess fuel / lack of air.

I am not familiar with which Shercos have a vacuum fuel pump. If yours has make sure it has a return line and that this is clear / functioning properly. Make sure your cold start enrichment is shutting off properly when turned off. Other than that is normal carb checks like float height, float valve cleanliness and operation. jetting etc. You don't state what oil to fuel ratio you are using. Empty you tank and refill with a mix ratio of 1 litre of petrol and 30cc oil and see if that improves running.

Check air filter is clean. Examine float valve parts with magnifying glass.

http://www.moto-parts.co.uk/sparkplugdiagnosis.pdf

http://www.totallyamaha.com/snowmobiles/aaTECH/sparkplug/Spark%20Plug%20Information%20%20Overview.htm

 

Please bear in mind that is impossible to give an exact diagnosis without having the bike to hand, an ignition fault is still a possibility.

The spark plug should smell the same or very similar to the exhaust.

Edited November 29, 20169 yr by dadof2

It's running too rich going by the colour of the plug and the other symptoms.Take it to someone who knows how to sort the carb out

Thanks for the advice from all of you I really appreciated it, i'll get stuck into it this weekend.

Edited November 30, 20169 yr by dwb5151

Pump diaphragm holed / leaking?

Did you get it sorted? Find what problem was?

I think i have found out what the problem was the pulse pump was pushing to much fuel through. I noticed this when a full tank of fuel wouldn't even last an hour.

Then the bike wouldn't start , so we changed the CDI box now it starts and run right, Thanks for the advice.