Motorcycle Repair: yamaha 400f dirtbike, spring retainer, cam lobe

Question
My dirtbike hard starting when I do get it to start it back fires and pops real loud and I can see flame coming out of the tail pipe. After it gets hot it dumps antifreeze out of the over flow hose. I can also only start it when the choke is on and it doesn't idle unless the choke is on. My question is how do I find the problem and where do I start?

Answer
Christopher, Start with the valve lash clearance. As the valves seat in you lose the clearance between the cam lobe and shim bucket. This makes them run rough and hot and almost impossible to start. Hopefully the valves are'nt burnt and have to be replaced. The cam has to be removed and the bucket lifted off the valve stem and the shim sets on the end of the valve held in place by the spring retainer. The shims will have the thickness marked on them unless they are worn then you will have to measure them with a caliper or micrometer. Also put in a new spark plug while you are at this to start out fresh. A;so just check the tightness of the head pipe to the cylinder if it sucks air between the pipe and the cylinder it will shoot flames mout the back also.