I have recently aquired a SR125 1993 & have a problem with it, sometimes it seems asthough it will lose power & just sputter & backfire when given some throttle, although will idle fine, its as if something is sticking, then after a while it clears itself & runs fine for a while then it will happen again, we have taken the carb apart & that is clean with all jets clean, replaced the fuel tap & pipe & spark plug. But still no joy. Should I now consider taking the head off & checking valves etc. I have taken a photo of the removed spark plug, as a complete newbie please be gentle. thanks http://www.flickr.com/photos/52004648@N02/4790885422/

Running rich clean the air filter . Read other posts & get a manual .