Anyone have thoughts just got a 85 250r rode it before I bought it. got it home tore into it put a new stock piston and boysen reeds in it, cleaned the carb, checked carb all stock jets settings ect. I put fresh gas with yamalube 2r 20-1 mix 93 pump gas. I rode it and it runs good very crisp throttle and idles good but after 30 min. it stops running it will only restart after putting in a new plug. runs anouther 30min. same thing. I'm not sure if its carb or ignition. anyone have this problem? Thanks

If it is basicly a stock bike with stock carb settings, odds are likely it is too rich somewhere.

First thing I would do is spend some time re-jetting.  Find a procedure online, buy a half dozen plugs and start checking the jetting circuits.

yea now thinking about it the guy I got it from was saying it fouled plugs and he switched oils. I think I am going to try to drain the gas and do a 32-1 mix with a br8eg it has a br9eg in it now. I have to start somewhere. thanks for the input

Figure out what mix ratio you are going to run, then jet.  i run 40:1 amsoil in my cr250.  My CR250 ran horrid with the stock jetting, it forced me to learn to jet.  I was always scared of jetting and stole/borrowed others jetting recommendations.  A well jetted bike can be amazing in comparison to stock jetting.

I did a new mix at 40-1 and put in a hotter plug and so far its running much better. I will try to ride some more then read the plug