Solved my problem thanks to 9000Whites!
Hi there, just to let you know my problem of carbonised sparks is solved. It was related to the valves that let fuel in connected to the floats. It seems there was some o-ring that were getting old and leaking fuel. So I just changed the o-ring of the needle valve and it's running great.
Tough it didn't solve the problem until I changed the oil... it seems since gas was leaking, some of it was getting into the base of the motor and mixing with oil (baaad). I had 4 liters of liquid in a base where you usually put 2.2 liters of oil ! (dam I'm lucky no real damage has been done) .
Thanks for those of you that helped me!
BeerBaron