So, I have had my 250f for a few years now and have never had nor do I currently have any issues with it. Just a lack of proper maintenance in checking valves on my part. Other than that I maintain properly everything else. I have feeler gauges ordered so I can properly check valves. Here is the question I am posing though. On the head there are etchings on it in 2 places which only has me curious on what was done to it by previous owners. I believe I am 3rd in line. I bout it from a young kid who stated it was too powerful, he owned it a few months and I bought it from his dad. I don't know prior history. On to the etchings... On the brake pedal side of head there is the etchings 5z84 right at the carb intake and at the base of head is the etchings 4zc-88. I have no idea if these mean anything. Can you fellow owners look at your head and inform me if these are on other bikes or if possibly mine has been worked on? Thanks you guys.