The problem I am having is that cylinder number 4 isnt working at about 3K and over. it idles perfectly well and drives normally upto 3k but after that it splutters and sounds like a missfire. I have checked all spark plugs and there is no oil or fuel present. Its not burning any oil at all and there is no smoke of any kind (black or white) leaving the exhaust.

I know I am going to have to take the head off to see whats wrong but for the life of me I can not work this one out.

I am using a Bailey Breather.