I recently acquired 1985 yfm 200. Choke cable was broke and fuel filter was in backwards when I got it. I have since replaced choke cable and turned fuel filter the right way, changed oil and filter, and replaced spark plug. When cold it seems to run rough and backfire. Once it warms up it runs good. It seems like the longer it runs the better it runs. Any advice on what is causing my rough run is greatly appreciated.

 

I would check your valve clearance, could be too tight, then when it warms up, everthing expands and gets withing spec. too tight will not allow full valve opening which would make it run bad when cold.