Luca,

I see BobF has asked you what engine but so far you are a no-show with a reply.

If you want others to help, you have to firstly show some interest and secondly appear to know what you are talking about so as to understand any information passed on. From where I am it seems you don't have a clue - correct me if I am wrong.

Thinking it has only got 3 valves is probably most of your problem and indicates to us that you couldn't adust them anyway even if we told you. Which begs the question, why are you asking?

Regards