Thank you for your recommandations, it's good for me to have your points of view in the balance.
@gs34 :
Last time i cleaned the spark plugs, they were cover with half an inch of this black residue. So i guess in the head and on the piston there is more of this black this than there should be.
@TQuentin1 :
I know the good way to do it is to do like you say. I would like to do like you say. The problem is i also have the cam chain tensioner problem and i orderer all the necessary parts at Herko and beside this i baught tools and with the taxes and shippment now it's a 3200 US.$ bill.