I have just done a restoration on a crf250r. Got it really cheap as the guy didn't have any money to fix it and couldn't start it. I pulled it right down, checked valves and clearances, new spark plug and air cleaner and changed the oils and filter. It then fired on the third kick and ran really well for a while. After I turned the fast idler off it idled fine for 20 seconds or so then conked out. Started every kick after that but doesn't idle for to long before it conks out again. Revs fine just won't stay running. Don't have a manual to do any troubleshooting, any suggestions? Could it have anything to do with the valves?